School Idol

The single most popular student(s) in school.

Beyond that, however, school idols can be a widely varying bunch. Though membership of The Beautiful Elite is generally mandatory, school idols can be academically accomplished (or not), great athletes (or not), or the nicest people you've ever met (or not). While more school idols tend towards being Aces than not, what really only defines this trope is massive in-series popularity; most common symptoms of which include being met with fawning adulation wherever they go, an Instant Fanclub or three, and/or a group of highly jealous and violently protective stalkers.

Storywise, if the school idol is a secondary character, he or she is likely to be a Love Interest, most likely in a Single Girl Seeks Most Popular Guy setting; if (s)he's the main character, the story usually revolves around his or her struggles to keep up the perfect facade.

This trope is frequent in Japanese media, where the character is often The Ojou, functioning as something of a School Princess. While the notion of "most popular student(s)" exists in the standard Western High School setting as well, Alpha Bitches and Jerk Jocks are generally portrayed in a much less positive light, and as such rarely have a similar Cult of Personality to the school idol. A possibly helpful distinction: is the student an idol (as in, is he or she idolized?), or "merely" wildly popular?

For other kinds of idols, see Teen Idol and Idol Singer.

Examples of School Idol include:

Anime and Manga

Fujiko Etou from Ichiban Ushiro no Dai Maou.
The Guardians in general and Hotori Tadase in particular in Shugo Chara.
Shino Amakusa of the student council is said to be one in Seitokai Yakuindomo.
Fruits Basket has Yuki, and his insane in-story fan club.
His and Her Circumstances has Yukino, who is definitely of the sort who has to work hard for it.
Tachibana from Ai No Kotodama (manga/movie) finds out later that he was considered a school idol.
Then there's CLAMP School Detectives, which takes it ever so slightly to extremes.
The guys from Yamato Nadeshiko Shichi Henge. Kyohei is a total jerk, but the other three aren't as prickly as he is and every bit as popular.
Kaname Chidori of Full Metal Panic! is explicitly said to be one as she is introduced.
Similarly, Rin Tohsaka is the most popular girl in school. This is achieved by being very good looking and keeping up an honor student facade, but she's actually quite a short tempered Tsundere (more evident in the original Visual Novel than the anime.)
Love interest Natsu in Baby Steps is referred to by the protagonist's best friend as the school's "Number 1 cutie." He and several other students then threaten the protagonist with bodily harm when they catch him talking to Natsu in the hall.
Hinagiku Katsura of Hayate the Combat Butler, it's even called out on, particularly during the Valentines Day and White Day arcs, and a lampshade is hung during her birthday arc.
- Maria and Athena Tennos used to hold this position before they graduated, Athena is even said to have shared the title with Hinagiku.
Haruka Morishima in the Amagami franchise. She even sounds like Hinagiku above.
Oniisama e... actually has three. The first is Student Council President Fukiko "Miya-sama" Ichinomiya, an elegant and beautiful and very unstable Ojou; the second is Fukiko's half-sister Rei "Hana no Saint-Juste" Asaka, a kind yet troubled Bifauxnen who's also a musician and ex-sportswoman; and the third is Kaoru "Kaoru no Kimi" Orihara, a sports idol Ladette and the Class Representative for Nanako's class.
Otome Katou from School Days, said to be most sporty girl at school.
Ai Ore Love Me features Akira, the "Princess" of the all-boys High School he attends; and his girlfriend Mizuki, the "Prince" of the all-girls school she attends.
Princess Princess features three boys who are idolized at their all-boys school as "Princesses".
Utena Tenjou, among middle schoolers, Touga Kiryuu among high schoolers.
Sakaki from Azumanga Daioh is fantastically popular because her loner tendencies make her seem aloof, which appears to be the entire basis upon which the other girls at school determine her to be "cool". Really, though, it's actually a case of crippling shyness and she doesn't much care for the attention; she prefers the closer relationship she has with the rest of the main characters.
He Is My Master has Sawatari Mitsuki, who works as a maid along with her big sister. She has bodyguards at school. Her master, on the other hand, is far from being popular.
Nogizaka Haruka no Himitsu has Nogizaka Haruka, who is an idol at her school. She's an otaku but hides it out of fear of a repeat of the humiliation she suffered at middle school, where she was an idol until then. Her little sister, Nogizaka Mika, who attends an all-girls middle school, is an idol there.
Gasai Yuno from Mirai Nikki is often described as the school idol, being on the ouside a pretty girl with excellent grades that is very nice to everyone arond her. The reader soon realizes the truth is a bit more disturbing

Film

Ferris Bueller is the definitive Western example. The entire school grinds to a halt when they learn that Ferris is "sick", and devote all their energies to making his life better. He's also beloved by all, regardless of their own place on the school totem pole:

The sportos, the motorheads, geeks, sluts, bloods, wastoids, dweebies, dickheads - they all adore him. They think he's a righteous dude.

Literature

Dear Dumb Diary has Angeline, who is seen by the narrator as a mixture of the Alpha Bitch and this trope. It's played almost completely straight in the eyes of the rest of the school.
- A lesser example is Chip, mentioned in the second book as "the cutest guy in school," with his dog being more popular and better-looking than most boys.

Live Action TV

Nobuta wo Produce is about a school idol who works really hard to keep up this image until he starts hanging out with those two weird kids, one of whom he is trying to turn into a school idol as well. At some point he ends up falling from his idol position spectacularly, but in compensation he learns The Power of Friendship or something.

Video Games

Yukiko Amagi is vaguely hinted to be one of these in Persona 4, but Naoto Shirogane actually makes the local news as the "Detective Prince". Amusingly enough, despite being a Teen Idol, Rise Kujikawa doesn't appear to become a school idol once she enrolls.
Mitsuru Kirijo of Persona 3. Granted, her status as something of a Yakuza Princess thanks to being the heiress of a Mega Corp involved in shady activities that makes generous donations to the school allows her to practically run Gekkoukan High regardless of what others may think of her, she still has quite a reputation and more than a few fans among the student body.
- Yukari Takeba is quite popular among the student body as well. Hell, she's even been called "the famous Yukari Takeba" in-game.
Da Capo has Kotori Shirakawa. She's explicitly referred to as this, and it's easy to see why: she's pretty (enough to win the school beauty pageant!), a good singer, and one of the nicest people a person could hope to meet.
Tokimeki Memorial 1 's High School, Kirameki High, has three School Idols dominating the campus: Saki Nijino, "The Idol of Kirameki High's Sports Clubs"; Mira Kagami, "The Madonna of Kirameki High", and Shiori Fujisaki, "The Superstar of Kirameki High". Their Iconic Items (Saki's home-made "NijiBen" lunch box, Mira's earrings, and Shiori's hairband), are, according to Info Man Yoshio, the most valuable items among the boys in the school, and getting one of them is considered as a feat and a proof of close relationship with the girl.
Amagami's very own Haruka Morishima who is also explicitly called, "Man-Killing Phenomenal Queen" in their school. She doesn't act like one though.