If the character's physical age is reverted to school age, they will have to go to school again, even if they're Really 700 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 if 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.
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. When she has a few seconds left to live.
Evangeline is an immortal vampire who was turned when she was ten. When she was defeated by the main character's father, he cursed her to live at a school as a form of Ironic Hell. The original plan was for her to be released at the time of her first graduation, but, well...
Nagi: Oh, Eva! You're still cursed? Didn't I come and undo— Eva: Shut up, you idiot. I know you forgot.
Chachamaru, who a) is a robot; and b) is only about two and a half years old; but attends school anyway. Of course, she was built in the first place to be Evangeline's servant at school.
Sayo, a ghost who can barely wander to the nearby convenience store because she's bound to the school. Eventually 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.
Subverted in xxxHolic with Watanuki. Doumeki tries telling him he can't just stay in the shop, he has to go to school. Actually, no Doumeki. Cause I've made an agreement to run this shop and live without aging until Yuuko returns, and I can't leave until that happens.
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.
Ano Natsu De Matteru: After a teenaged Human Alien crashes on Earth, what's the first thing she does? Enrolls in High School, of course. Though pretending to be a foreign exchange student does provide Ichika with a useful cover, an excuse for her odd behavior, and ready access to the boy she likes.
A Certain Magical Index has this mixed. Touma and Tsuchimikado both still attend school despite all their adventures, but many teenaged or child characters do not due to various circumstances, such as Index, Accelerator, or ITEM's members. One scene in New Testament has Yoshikawa and Accelerator discussing enrolling Last Order in school (to which she strongly objects) in order to give her a somewhat more normal life.
In Cardcaptor Sakura, Eriol is several centuries old because he is a reincarnation of Clow Reed. However, he stopped his aging process, so he still has the body of a boy about 11 years old, and enters Tomoeda Elementary as a new transfer student. Justified, as he needs to get close to Sakura and Co.
In Haiyore! Nyarko-san not only have Nyarko, Cuuko, and Hasta already graduated from Space College by the time they show up on Earth, they're also LovecraftianCosmic Horrors of uncertain agenote As in, when Mahiro tried to learn Nyarko and Cuuko's ages, the girls physically restrained and threatened him into dropping the subject — but since their human forms are teenagers, they go to high school along with Mahiro.
Averted prominently in Macross 7. Mylene is only fourteen, but never attends school, the reason being that she is a successful rock star and her career takes up all of her time. She does briefly pretend to be in school when she first meets Gamlin, but drops it when he finds out that she's in a band.
In Neon Genesis Evangelion, Asuka claims to have already completed her college education, but still attends school with Shinji and Rei. Judging by how effortlessly she helps Shinji with some of his homework, she may not be lying about this. When Shinji questions how she can possibly be getting bad grades, she replies that she is still a beginner at kanji reading, so she can't actually read most of the questions on their tests.
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.
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.
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.
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.
Starts out justified in Jinx High by Mercedes Lackey. The villain Grand Theft Me'd her daughter, who was already in public school. Then the justification falls apart when she thinks about how previously she'd been able to claim her current body was being educated at home by private tutors — homeschooling is legal in Oklahoma.
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.
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.
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 it has been suggested that she be put in a remedial class.
Subverted 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 adviser). 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).
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.
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".
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.