I don’t know why is everyone assuming Shuu is a girl? Pretty sure that is a male name.

idk a better way of phrasing this but you can clearly see she has a female anatomy once she takes her hoody off

Maybe shuu is a nickname

I'm fairly certain Aya would address them as "Shu-kun" and not "Shu-chan" if they were a guy, right?

Depends , some use inverted pronouns (endearment names) in a mocking/jk way

lol I reread the chapter and I can see it now. Still weird because I swear Shuu is a male’s name. Maybe it’s shortened, or perhaps it’s her surname.

There’s no strict rule like that. -chan can be used with a male friend or kouhai close to you (in a cutesy way) while -kun can also be used with girls especially the energetic and sporty type.