Steam does reject games with high-school settings for characters who they think are minors.
That may have to do with something else.
AFAIK in Japan they are extremely tight on having the sexual bits censored (though some censoring I have seen on some comics is a couple of black bars, so you still recognise easily it is a dick), but it seems at least insofar as the exact age is not explicitly mentioned and it is a manga, a game or at maximum an animation (not real stuff with real people), they don't intervene. Hence, the high school setting is fine for them, even if it cannot be guaranteed that the characters are not younger than 18 (in some countries, including Japan, high school ends at 18-19, so even without having to repeat, a high school student can be 18 or even 19).
In at least some, probably most, countries in the "West" either the law directly has become, or has been interpreted, as such that any depiction in any way, even if you draw it by hand yourself, of anybody that can be understood as being less than 18 of age (independently from absence of any explicit indication, or possibly even disregarding any explicit indication) in a sexual activity is considered pedopornography.
In one country I know of, it is not directly the law that says that, but a judge interpreted it that way so it could condemn some 14 years as pedopornography criminals for having used "generative AI" to create fake images/videos of some classmate or schoolmates.
I don't know all the details of that case so I cannot have a firm opinion, but honestly, I think it was too hard to condemn basically for life (because "pedopornography" is not "slander" or something similar, is something that will stay with them for the rest of their life, and there will not be any details to explain the context) adolescents for having done something (thinking about sex and girls and commenting/thinking about girls close to their age and that they see every day at school) which is normal for adolescents, though I understand the problem deriving from their use of the tools instead of just fantasizing, and the damage that can have been felt by the girls.
Apparently in some state in USA (or all of USA ? Don't think so) at a certain point they had gone further, completely forbidding without exception any depiction of any form (so, including hand drawn, renders, etc.) of people below 18 (or 21 ?) where could be seen the sexual bits, even if they were not depicted doing sexual actions - some content creators for Daz had indicate that a couple of years ago as the reason for some character having alternative textures.
If that is true, what happens if two adolescents below 18 do "sexting" with MMS with racy pictures (as it happens nowadays) in those States ?
Technically, it seems if that law is taken literally, both the girl taking the picture and sending it, and the boy receiving it, are guilty of pedopornography, it is irrelevant that she is the subject of the picture and he is her boyfriend (well, a bit like in California, even if they are both below 18 and it is consensual, if they have sex they are both criminals, though in that case is lowered to a misdemeanour).
What would happen in the same States if somone has a sexual education book, or a book for someone studying pedriatics, showing the body of an adolescent boy and/or an adolescent girl with the penis and the breast and pelvic area, even if they are not photographies, but rather sketches and so not fully realistic ?
It seems if that law is taken literally, they would also be forbidden as pedopornography, and the adolescent that has the sexual education book and his parents (if he did not buy it himself) and the medicine student would all be pedopornographers, which if confirmed, IMO is an example of starting with good principle (protecting the small ones) and going all the way to complete idiocy.
In that interpretation, a book of sexual education I had when I was adolescent, which was absolutely not about sex (I mean, was explaining pregnancy, had a sketch of fallopian tubes and ovaries, and showed separate sketches of a boy and a girl in puberty to show and explain the physical differences that start to become visible when puberty kicks in, but it was all very medical and was not describing in detail, even less depicting, sexual intercourse) would be considered pedopornography.