School Clubs Are Serious Business

Students get involved with extracurricular activities for all sorts of reasons. They let you network with other people who share your hobbies, they let you use school resources for fun, and they look great on a college transcript. While there are some exceptions, like performance clubs with recitals and organized sports, most clubs are just afterschool activities pursued casually by their members.

That's not the case with these clubs, though. In the more benevolent cases the club is simply treated as a full-fledged social institution, with stereotypes about its members and strict rules about its conduct. A powerful enough club is a political force in the school, potentially wielding power on par with the Absurdly Powerful Student Council. In the worst cases the club may actually be a cult engaged in nefarious deeds, and those who quit may have to be ... silenced.

Note that these tropes can be different types of "serious business". It can be very exaggerated, like clubs determining all of your relationships in high school, or completely bogus, like harboring a plan to stop an apocalyptic prophecy. The trope may split if there are lots of examples.

Related to Absurdly Powerful Student Council. Subtrope of Serious Business.

Anime and Manga

 * In Haruhi Suzumiya. Haruhi was just another high school girl who wanted to meet aliens, time travelers, sliders and espers... AND ACTUALLY MEETS THEM.
 * Parodied in Keroro Gunso. Fuyuki starts the paranormal club, but only has one other member in it (and she only joined because she had a crush on Fuyuki). The school newspaper also discovers the aliens' existence, but nobody believes them because they're a school newspaper (and because Fuyuki managed to confiscate their evidence).
 * In Rosario + Vampire, the protagonists are all members of the newspaper club (Tsukune origionally decided on joining the swim team instead, but that left Outer Moka heartbroken because, well, vampires and water...), which spends much of its time investigating other clubs. The newspaper club has a rivalry with the Public Safety Commission (another club), with effective control of the school at stake. There's another club in the anime that consists of three lovesick losers who engage in such activities as creating a harmless Stalker Shrine.
 * Genshiken: The people who decide which clubs are official are cracking down on those that don't do anything, because there are so many clubs that even the Genshiken's remote space is coveted.
 * Magic Users Club: Even though they actually are quite active, the magic users have to share meeting space with a different club, which causes some conflict.
 * K-On!: Invoked in the manga, when Nodoka discovers Yui isn't in a club so late in the semester. Nodoka ponders, "So this is how a NEET begins", causing Yui to freak out.
 * In Code Geass, Absurdly Powerful Student Council President Milly is able to get every club on campus looking for Arthur by promising them increased funding, in addition to the kiss with a council member. At the end of the episode, Lelouch asks Milly to let the Japanese Suzaku join the student council by mentioning that students must be part of a club.
 * Invoked and Lampshaded in Twinkle Saber Nova, where students can get away with literally anything (like trying to take over the school, or demolishing half of it in battles involving Powered Armor), as long as it's a club activity. And to make it a club activity, they only need three members to register their "club".
 * The Ouran High School Host Club consists of some of the richest and most popular boys in the school, and they have the headmaster's ear and influence over seemingly every facet of school life. Fortunately they use this power almost exclusively to stroke their own egos.

Live Action Television

 * The titular club in Glee is VERY unpopular, and anybody in it are social outcasts, a plot point in the series. They also occasionally start spontaneously singing in hallways, drawing the school's attention.
 * In Buffy the Vampire Slayer, Buffy tries and fails to join the cheerleaders. Someone else is so intent on joining the team that she's trying to kill people to get on, and so does Dawn, though there was a different spell in there.
 * In one Blue Bloods a sinister college club develops the ceremony of ritually "hunting" female students as if they were trophies. One member actually volunteers to rape his girlfriend on the grounds that he would be more "humane" then the other boys. It never seems to have occurred to him to warn his girlfriend or go to the cops until the cops actually caught him. His school club was just too important.
 * In Community, Greendale Community College is full of these, ranging from the debate team and the glee club to the Model United Nations. All of them turn their pet interests Up to Eleven. The study group itself qualifies, despite being informal; it has a waiting list for membership and the Dean treats them as a school power faction.

Video Games

 * School clubs give gameplay benefits in Disgaea 3: Absence of Justice.
 * In Persona 3 and Persona 4, joining clubs can start several social links, and the characters involved in the Social Links can be altered depending on which club you join.
 * School clubs are an extremely important part of the Tokimeki Memorial series. Most of the winnable girls being members of specific clubs, joining their respective clubs is the only way to get some of their special Events and CGs. Also, School clubs give you powerful special attacks for battles; perfoming well in the club during the School Festival and, in the case of sports clubs, winning matches and Inter-High tournaments net you big Relationship Values towards the girls; and becoming the Club Master of the Science Club allows you to unlock the Shooters mini-games.

Western Animation

 * The Grim Adventures of Billy and Mandy had an episode where the students joined school clubs. Mandy joined the Secret Snake Club, who believes in a snake that will eat all of the cool kids. Billy joined the Secret Service club, and discovers the knitting club is being used for criminal intent.
 * Fillmore runs on this trope, along with Absurdly Powerful Student Council. The Safety Patrol has the power to arrest and punish students on their own, what else needs to be said?

Real Life

 * When it comes to the "god" thing, clubs can get political.
 * Back in the sixties, the secular movement was warming up, so religious groups got in on the ground floor and started religious school clubs. The courts were iffy, but ultimately decided that the first amendment meant that the groups, student formed and led, had to be allowed.
 * Fast forward 45 years and the atheist/secular movement is taking advantage and forming secular/atheist student groups. Thanks to the Supreme Court's earlier ruling, they can't be opposed.By the petard are you hoist. The drama is still... intense. Death threats are the norm.
 * This is Truth in Television in Japan, since it can be a key factor during secondary education.