I get why you say that but I think in this case it works because they aren't just your "average" high schooler. They're all from yazuka or powerful backgrounds. SHouma and Kirishima have both sad pretty shitty pasts from our limited knowldge and they have all grown up around lots of violence and have done/seen stuff other kids their age wouldn't have. It's only normal that they are much more dark and "mature' than other people in their age group. It's similar to how tons of fantasy/historical/grimdark YA novels have protagonists that are similarily in their teens but in no way act like a typical high school teenager.

Am i the only one who can never see them as highschoolers? I mean i know they’re one but no matter what in my head it just wont fit lol idk if that make sense