While due to the nature of anime as tending to depict characters with younger looking facial and bodily proportions means that we tend to prefer to assume characters to be over the age of majority without other indication, schoolgirl uniforms present a particular difficulty. Yes, it’s a common fetish, especially in hentai, and in theory there’s no direct reason to assume the character is actually a schoolgirl rather than someone roleplaying, as is common, see for instance !schoolgirlskirts. However, depictions in a school get more complicated, especially those where an apparent authority figure such as a teacher is involved. For the sake of drawing a wide berth around potentially illegal content, the rule is as follows:

Depictions of schoolgirls are allowed, however usual rules about presuming age of majority are more tenuous. Flat-chested / petite art will be judged more harshly by the mods for potential illegality. Any indication of the character depicted being anything other than final year will be treated as proof of underage content. Any depiction of sexual activity with a teacher or other apparent authority figure is banned. Any depiction of a schoolgirl in a school setting is banned.