Ok, so you want to watch a new anime, but you don't know if it is going to be good or not. Your first quality check should be to look at whether the main characters are adults. Having a cast of students doesn't make an anime automatically bad, but an all-student cast is definitely a warning sign. Don't believe me? Let's walk through some examples.
By nature of the post, this article will contain heavy spoilers to multiple popular anime. If you don't like that, simple - don't read it!
Code Geass: The plot really has to twist itself into knots to explain how Lelouch's student life and his alter-ego are not at odds with each other. Why is Kallen never at school? Well, she's the daughter of a rich bureaucrat who can do whatever she wants. Why do her parents not question the fact that she's never around? Eh, IDK, they hate her or something. Why is Lelouch never at school? Well, he's a rich bureaucrat who has no parents. For some reason he's the son of a king but he went into hiding and everyone is for some reason completely OK with this and nobody ever tries to find him. Why is Suzaku never in school? Well, he got drafted into the army for literally no fucking reason. Remember, the whole plot point about him being a super special pilot and the military needing his help happens at like episode three. At the beginning of the show, he's literally just a teenager who happens to also be in the army. (Which is not, like, completely unheard of. But then why does he start attending a school afterwards?)
The anime does show Lelouch actually going to school and doing high school activities, but it all seems pretty aimless. Nearly all of the time spent at school could be removed and the anime would be like 5 episodes shorter and lose very little.
Grade: C, anime would be much better if the characters were adults, but there was at least some attempt to realistically depict them as students.
EDIT: Actually in episode 21, both Lelouch and Suzaku get shit from their teachers for the fact that they're literally never in class. Notably Kallen is absent even though she's also never in class, but whatever. Raising the grade to C+ for realism.
Gleipnir: The main characters all live alone despite the fact that they are ~16 and have no money. Also their parents are dead or permanently absent. There are a plethora of excuses for why all the underage characters are running around with zero adult supervision and zero concern from their school. The fact that they are children has absolutely nothing to do with the plot, and they are basically never seen at school outside the first episode. Why are all these children completely unsupervised in a first-world country!?
Grade: D. The anime would be significantly improved if all the characters were adults. Also the anime heavily sexualizes children which is creepy.
Fate/Stay Night: Let me guess - multiple underage characters live alone because they have no parents, and there's basically no adult supervision to explain why they spend so much time outside of school.
To be fair, F/SN does mention that characters are sometimes at school, and there are scenes of them being at school, which is more than we can say about Gleipnir. There's even explanations as to how Shirou and Rin have enough money to live alone, the former needing to work part-time to earn money and the latter being a nepo-baby.
Grade: C+, anime would be improved if the characters were adults, but just in an average way.
Made in Abyss: The characters are only children because the author is a degenerate pervert. Grade: F, the anime would be significantly better if all the characters were adults and the author was in jail.
Beastars: This is a really interesting one. The characters start off in school, and there is actual plot-relevant things happening in school. Those plot points could happen elsewhere but the anime would lose a little bit of its charm. Questions about adult supervision are somewhat dodged by the fact that it's a boarding school, a concept which for some reason is heavily under-utilized in anime featuring underage characters.
However, at some point in the manga (anime not caught up yet), two of the main characters leave school, and the events of the plot start making much much more sense. (Well, as much sense as it can in Beastars, whose plot is royally fucked from the get-go, but not for this reason.)
Grade: B, depiction of underage characters is actually pretty sensible.
My Hero Academia: I had to include this so you know I'm not cherry-picking. (At least, not egregiously so.) The anime is centered around an academy for people learning to be heroes, so obviously school is a big plot point. The anime shows the main character having parents and there's a logical explanation for why he is sent off away from them. The anime just wouldn't be the same if the characters were adults.
Grade: A+, anime where underage characters at school is completely OK.
Marwaru Penguindrum: I had to include this so you know I'm not completely biased, as this is my favorite anime. School has little to no relevance to the plot. Characters live alone with no parents or adult supervision, et cetera et cetera.
In defense of Penguindrum, there are a couple of things which set it apart from other run-of-the-mill characters-are-in-high-school-because-reasons. Firstly, there is a very plot-relevant reason that the three main characters live alone without parents, and their parents are discussed throughout the anime. Secondly, the anime shows that the underage characters are struggling to do this alone, that their extended family is worried about it, and that their financial situation is tenuous. This is an added bit of realism which, while appreciated, doesn't save some of the more questionable explanations as to why nobody ever attends school. Finally, there are main characters who are actually adults, and adults with jobs that allow them a certain degree of flexibility in their hours, so it makes sense that they are galavanting around doing all the shit they do.
Grade: B-, could be a lot better if (all) the characters were adults.
Fullmetal Alchemist: The characters are underage, but in-universe they aren't really required to be in school. The same trope of parents being dead and/or gone is there, but at least the father comes back and in a way that's highly plot-relevant. There is at least one underage character who is like, "No, seriously, I have to stay at home and learn a trade because I can't just leave my parents and go running around having adventures."
Grade: B+, there's no reason for the main characters to be children but there's also no reason for them to not be adults.
Highschool of the Dead: Ok. Look. I know this is not supposed to be a serious anime. But like, come on. The only reason the characters are in high school is because that's the title of the anime. Grade: F.
Death Note: Honestly, pretty good. Similar to Beastars in that: the main character is realistically depicted as having a family and going to school like a normal person, the fact that he's a teenager doesn't help the plot but it doesn't necessarily hurt it, and eventually he graduates from school as an actual adult with a job that is plot-relevant and gives him time and an excuse to run around doing the stuff he does. Grade: A.
Kiznaiver: Doing medical experiments on children is not OK, and the excuse that "they have to be young because science" is lame. Most of the drama in the anime is completely boring high school bullshit that nobody over the age of 20 would care about.
Grade: D, the anime would be significantly better if the characters were adults.
Cyberpunk: Edgerunners: Main character is actually depicted as going to school and having a family, but of course the family dies because plOT ReASoNs and you have a bunch of teenagers running around doing crazy shit and nobody seems to care. However this is pretty in-line with the established cyberpunk universe, and the plot isn't significantly impacted by the fact that the main characters are underage.
Grade: B+, no real reason for them to be children but also no reason for them to be adults.
Tokyo Ghoul: Character is underage, spends zero time in school, lives alone, nobody seems to care. Take one guess as to what I'm going to say about this. Grade: C.
EDIT: I completely mis-remembered the beginning of Tokyo Ghoul; Kaneki is actually 18 and a university student, which means it totally makes sense that he lives alone and is never actually in class. (Lord knows I was skipping class a lot at the age of 18...) Raising the grade to A.
Kill la Kill: This is a tough one. On one hand, it makes very little sense that the characters are all underage. But nothing in this anime makes any sense so that's all par for the course. The anime would be fundamentally different (in a bad way) if it wasn't in a school setting, as the school itself is a core piece of the plot. In addition to that, at least one character is depicted as having a family, parents, and motivation for staying in school - that is, not spending the entire show running around outside of school without any adult supervision.
It's hard to recommend this anime to people because it sexualizes children which is uncomfortable. But most of that is for gags and not meant to be taken seriously. Chalk it up to Japanese humor?
Also, at least one character is actually an adult (+18 years old) but is still attending the school as a student, which, ... look man, I don't even know.
Grade: B+? A-? I guess?
Bakemonogatari: Why? Just... why? There is literally, literally no reason for the characters to be students. They are depicted in school exactly once, and there is no plot-relevant reason for it. Grade: D.
Puella Magi Madoka Magica: This anime thrives off of stereotypes and also subverting them, so it kiiiind of makes sense that some of the main characters are underage, otherwise it wouldn't be a Magical Girl Anime™️. However, it would make a lot more sense if they were adults. In fact, one of the characters is an adult, apparently unemployed, and it's thus reasonable that she has all this free time to go hunt demons and shit. Grade: C.
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In conclusion, I hope you now realize two important things: that anime featuring adult characters have (in general) better writing, and many anime with underage characters would have a tighter plot and just overall make more sense if the underage characters were adults.
