I think it's more concerning your mind went there. Bro was just trying to finish his final grade in high school yet ur mind goes to a weird place. That's what doesn't sit right with me.
Too many stories on this site go there. A lot of bl authors write about minor/adult relationships and I was wary of that trend while reading this story. It's a red flag is all.
I agree. I met and had classmates back in my junior and senior highschool that is a year or even as old as Isshin. They had personal problems, like they lost both their parents and as the eldest, they're subjected to be their younger siblings' only guardians. (Note that not everyone's relatives cared enough about their extended family.) After raising them (younger siblings), they wanted to at least finished highschool as one of their accomplishments in life. Often in my country, (and I'm sure everywhere around the globe) hopes to at least graduate in highschool. It's also like fulfilling their promises to their late/parents. It's sad that some people would look through deeper into this... I know this is fictional and we're in this genre but this truly happens in real life. Everybody despite gender and age have the rights for education!
But I just couldn't get past the premise, no matter how hard I tried! An adult should not be attending school with kids! The characters were sweet and they didn't do anything unsavory, but something about it just doesn't sit right with me.