Compulsory School Age

If the character's physical age is reverted to school age, they will have to go to school again, even if they're Really Seven Hundred Years Old or have an IQ of 250. Either nobody must know, or they're some kind of high school spy, or there's just no explanation whatsoever except that kids have to go to school.

Also, aliens or inhuman beings that look like teenagers will go to school in the appropriate year for their apparent age. Nobody ever takes or needs remedial classes, nobody skips years, and nobody drops out.

This probably plays into certain insecurities ("back in school" is quite a frequent nightmare from empiric evidence). It may also have to do with children who are visibly not in school being seen by Moral Guardians as a "bad example". Apparently it hasn't crossed their minds to make up a story about being home-schooled. Probably because you believe everything you see on TV, home-schooled kids are weird, socially-inept freaks.

This is, however, Justified in many countries where homeschooling is not an allowed form of compulsory education. As a result, if a person needs to maintain that Masquerade, they need to go school. In some cases, parents in these countries will be served with a court order because they tried homeschooling their kids, and cases where such kids were taken away Elian-style is not unheard of.

May be used to set up a Wake Up, Go to School, Save the World premise.

Anime and Manga

 * Fullmetal Alchemist has Pride,  who has the appearance of   but is , and posesses.
 * Variation: In Ojamajo Doremi, Dokkan has two-year-old Hana-chan undergo a Plot-Relevant Age-Up. Off to the sixth grade for her! (Justified in that it was what she wanted in the first place—to be with her moms.)
 * Bleach has many of the Shinigami show up at Ichigo's high school to check up on him, much to his horror. Their bizarre physical appearances (notably Ikkaku, who's bald and carries a bokken around with him because he can't have a regular katana), combined with their school uniforms, causes everyone to automatically assume that they're delinquents.
 * Earlier in the series, Rukia starts attending Ichigo's school after he takes her shinigami powers. It's somewhat justified though, as A) she's living in his closet and can't exactly hide in there all day, and B) she needs to keep an eye on him and instruct him how to fight hollows.
 * She isn't mistaken for a delinquent, though, because she A) Follows the dress code (except for once wearing her winter uniform after everyone else changed), B) Is intent on hiding her secret identity (going so far as to comically threatening Ichigo with a note written on her hand) and C) Actually looks likes like a teenage girl. It helps that she's relatively short and flat-chested.
 * Played straight Code Geass with Suzaku who despite being in the military is ordered to by Princess Euphemia to go to school at the same time.
 * In the Alternate Continuity manga spinoff, Euphemia goes to the same school as everyone else, despite being the Viceroy of the nation in which they all live.
 * Suzaku also suggest to Euphemia she could join the school as well.
 * Literally "compulsory" in Mahou Sensei Negima with Evangeline, an immortal vampire who is cursed to live at a school as a form of Ironic Hell.
 * There's also Chachamaru, who a) is a robot; and b) is only about two and a half years old; but attends school anyway. Somewhat justified in that she's Evangeline's servant, but still...
 * Also Sayo, although it's kind of justified as she's bound to the school until Evangeline and Asakura give her a doll to possess that allows her to leave the campus.
 * Astro Boy is sent to grade school with human children of his apparent age despite being a robot with a super-advanced AI. Sometimes this is explicitly said to be for the purpose of socialization, sometimes not. Subverted with Astro's robot "parents", who are also sent to school in a lower grade because they're actually younger than him.
 * Burst Angel has Meg and Jo infiltrating a boarding school looking for Body Horrors.
 * Subverted in ×××HOLiC with
 * In Space Pirate Mito, the titular character is an alien who is Really Twelve Thousand Years Old, but looks like a third grader. While spying on her half-human son in high school, she's caught by a teacher and sent to elementary school, and she continues to attend, as seen in later episodes.
 * Magical Girl Lyrical Nanoha A's: Averted in the first Sound Stage, where Hayate asks Vita if she would like to go to school, but Vita says she's not interested.
 * Shin'ichi Kudo from Detective Conan is already a teenager. Then he gets turned into a little kid and ends up having to go back to elementary school.
 * This is also the case for Ai Haibara, 18 years old, and the developer of the aforementioned Fountain of Youth.
 * Averted in the manga Black Jack with Black Jack's assistant/adopted daughter Pinoko, who is technically eighteen, but has the body of a girl no older than five or six. In the chapter "Pinoko's Challenge", her attempts to get into high school (she suffers nasty psychosomatic illness from the stress of entrance exams) and kindergarten (she gets kicked out after throwing a violent tantrum) both end in disaster.
 * Elsie from The World God Only Knows goes to school with Keima despite being a demon girl who's over 100 years old.

Comic Books

 * Raven from Teen Titans got placed in the body of a teenager. Back to high school you go!
 * In this case it was intentional—she wanted to get a taste of the "normal" teenage life she hadn't had originally.
 * Miss Martian also from Teen Titans inverts this in that she wanted to go to high school to learn how to fit in, but couldn't until she had forged the documentation.
 * Superboy was removed from the accelerated growth chamber he was grown in at the apparent age of 16. He had to take a test to see if he needed education or could just rely on his implanted knowledge, and then skipped it to save Hawaii from the Silicon Dragons. High school it is.
 * And then a few incidents convince the people involved that public schooling and superheroes without secret identities don't mix, so he starts getting home-schooled. When he later gets a proper secret identity, he starts attending high school, but it's never mentioned whether he get a G.E.D or the like in the time between.
 * Justified and subverted in The Adventures of Barry Ween Boy Genius, whose titular hero chooses to remain in junior high, because he's calculated that the socialization he receives there reduces the probability of being driven wholly insane by his constantly-increasing intellect from Nearly Inevitable down to Merely Very Probable.
 * Happens in an issue of Hack Slash, where Miss America On Phlebotinum Gone Horribly Wrong joins a boarding school with a secret Les Yay club looking for human blood to keep her body young and beautiful.

Fan Works

 * Sometimes seen in fanfics, especially of the self-insert variety, to allow the Author Avatar to interact with the main cast of a high-school/college-set show or vice-versa.
 * Played Straight and Averted in the Mega Crossover shared-world story My Apartment Manager is not an Isekai Character, Depending on the Writer. Rob's stories have his characters place out of high school if they're smart enough; only half of these characters continued to university. On the other hand, Brent's stories have the youngest Undine forced to attend high school despite having been about to graduate in her own universe – at least it's only part-time. And a secret no one suspected is revealed when the girls of Kickstand Cottage discover that one of their number is not required to enroll at the local high school.

Literature

 * Alice, Jasper, Rosalie, Emmett, and Edward from Twilight all go to school, though Rosalie, Emmett, and Jasper eventually graduate. Over the years, however, all of them have done this repeatedly. This is supposedly necessary because the younger they claim to be at the beginning, the longer they can live in one place without arousing suspicion because they don't age. Plus, it just looks weird to have a couple move in with their 5 barely-adult kids. But if the kids are still minors? Not odd at all.
 * Of course, the constant lies regarding sunny days and feeding field trips make it seem like it would be easier to say they were being homeschooled.
 * It would also let them stay in one place longer, as they would be monitored less and therefore the fact they weren't ageing would be less obvious. Less interaction with others would also both lower the chance of their secret being discovered and mean they wouldn't have to work so hard to avoid biting people.
 * In The Saga of Darren Shan, when Darren attends school to avoid drawing undue attention to himself.
 * Despite being a supergenius (and an evil one at that), Artemis Fowl's parents insist he attend St. Bartleby's something-or-other.
 * He didn't go to school in the first book, when his mother was completely insane. Later on, he was made to go back, which makes sense from his parent's point of view as they probably want him to be normal (him mother even forced him to ''wear a T-shirt). He still found it annoying.
 * Alex Rider, because it wouldn't be good if people found out he was a super-secret spy.
 * In The Dangerous Days of Daniel X, Daniel tries to avoid this by using his powers to create clones of his parents to tell people he is homeschooled, but he does go to school eventually for a short time.

Live-Action TV
": Are you sure you want to do this? : [looks back at a pair of cheerleaders] Yeah, I'm sure."
 * 3rd Rock from the Sun's Tommy is Really Seven Hundred Years Old, but he got the teenage body, so he has to go to High School. In fact, he was specifically put into a teenage body to go to High School. (He was awfully ticked about that, though.)
 * In fact, in an early (perhaps the first) episode, he explicitly states that he's older than Dick and wonders why he has to be the adolescent.
 * Anya from Buffy the Vampire Slayer, because she took on the guise of a schoolgirl to approach Cordelia and got stuck in it.
 * Jack from Stargate SG-1 chooses to go to school, prompting an exchange like this:

"Troi: You could return to the Academy. Take another degree. Brush up on your Latin. Picard: And be Wesley Crusher's roommate?"
 * Unfortunate Implications galore.
 * In 21 Jump Street, the cops are still adult age, but they look like high schoolers, so they have to go to high school to fight crime that happens in/around high school.
 * Referenced in the Star Trek: The Next Generation episode "Rascals", where people turn into kids:


 * The 2002 TV series That Was Then and Do Over both featured protagonists in their 30s who were reliving their high school years.
 * Lampshaded in Young Dracula, where the Count has no intention of sending his children to school until a social worker comes by to insist.
 * By Series Three, and post-Time Skip, the Count has come around to seeing the advantages of sending his children to school. He insists that Ingrid attends school so that he doesn't have to see her all day... despite her being twenty by this time.
 * Cameron attends school during the first season of The Sarah Connor Chronicles, although this was justified by her need to be close to John as much as possible.

Video Games

 * Eve from Parasite Eve 2. Clone of Aya, looks like an 11 year old, in the good ending ends up going to school for children of her apparent age.
 * Not that she learned much from her former caretakers besides how to control genetically-engineered monsters and stand perfectly still in the wake of poison gas.
 * Aigis from Persona 3 goes to the same high school as the rest of the main cast (minus Ken) despite being a Robot Girl who fights Shadows. Somehow, no one else notices that she's not really human. This is likely a case of Weirdness Censor, because it is lampshaded by the teacher who reads about Aigis being an "addept humanish weapon" and immediatly dismisses it as a mistake. It's also half her idea, due to her obsession with the Protagonist. It is also mentioned she has no knowledge about the past 10 years (she was deactivated during that time)
 * Oh and she does poorly on the first exam she takes, so going to school might be a good idea regardless of her age.
 * In The Sims 2, if your Sim's age is reduced, either by cheating or legitimate means, to the point that they regress back to the "teenager" life stage, they'll have to go back to school-even if they were gainfully employed as an adult.

Web Comics

 * El Goonish Shive:
 * Grace's first day in high school provides an aversion. She's having a hard time due to her genius-level intelligence and lack of real-world knowledge, and in the most recent strips it has been suggested that she be put in a remedial class.
 * Ellen have to go through the high school right after living through it with her Alternate Self in the Second Life. Though history and some of biology are not the same.
 * Subverted brutally in Sluggy Freelance "Torg Potter" stories, in which full-grown twenty-something Torg gets dragged to a school that awfully resembles Hogwarts. He isn't turned into a child, and promptly finds himself able to do whatever the hell he wants due to being both larger and smarter more experienced than all the students. His greatest foe winds up being the devious headmaster.
 * Sissy in Umlaut House 2 is 16, but is a brilliant (if unstable) science prodigy. Just before finishing her doctorate, she becomes a Mad Scientist and has to be stopped by the protagonists (one of whom happens to be her mentor/grad advisor). Her father decides that she's spent too much time away from others her own age and enrolls her in high school to get her better acclimated to mainstream society (it doesn't work).

Web Original

 * Whateley Academy has a few students who were originally well beyond high school or even college age: Sara Waite was once a best-selling author and Caitlin used to be an instructor at the school and a former Marine.
 * In both of those cases, part of the in-world justification is the need for a safe hiding place (it seems that the word 'safe' has a different meaning in the this world...) where their apparent age wouldn't be a problem. However, this same logic applies to Samantha Everhart, except that she appears to be over school age, so she became a member of the Security force instead; why Carmilla and Eldritch needed to be students instead of, say, instructors, was always a bit shaky, especially in Sara's case (she's perfectly capable of taking on an adult form). Added justification does come from the need to master their new abilities (and hormones), however.
 * Played Straight and Averted, Depending on the Writer, in My Apartment Manager is not an Isekai Character. Rob's stories have his characters place out of high school if they're smart enough; only half of these characters continued to university. On the other hand, Brent's stories have the youngest Undine forced to attend high school despite having been about to graduate in her own universe – at least it's only part-time. And a secret no one suspected is revealed when the girls of Kickstand Cottage discover that one of their number is not required to enroll at the local high school.

Western Animation

 * Inverted in My Life as a Teenage Robot, in which, despite being mentally and physically a teenager who happens to be made of metal, Jenny is assigned to a class matching her chronological age and has to survive a day in kindergarten. She gets to go back to high school when her 'mother' tells them "she was designed as a teenager".
 * Jimmy Neutron: Jimmy is most obviously a scientific genuis, with an IQ well over 300, and still goes to school to learn about things he most likely already knows.
 * In one episode, he is offered a college scholarship (and later a teaching job), but goes back to fifth grade at the end because he would rather be with his friends than with his intellectual equals. After that, it's a Justified Trope.
 * Occurs in an episode of The Life and Times of Juniper Lee when Jasmine experiences Fountain of Youth syndrome.
 * Invader Zim. Zim is older than any human alive, probably hundreds of years old, yet he attends the local School Skool, in his efforts to learn about humans by observing their human worm babies.
 * Averted with Wade in Kim Possible. Justified by a mention that he's already finished high school and college in a few months thanks to his genius intellect.