oh i do have an answer for this. of a sort. so as a japanese highschool kid, koharu actually isnt supposed to have any piercings on during school. like ANY. even if theyre the lil simple round ones and not just the mega dangling ones.

the fact that class representative and department chief yanagi koharu does wear one along with her seiran jersey is an insane amount of delinquency that has never stopped being funny because the fact that its ONLY with the jersey means that shes been specifically putting it on During School

(koharu's dangling earring during revues gets a free pass because A) seiran trio matchies outfit do not separate and B) revue outfits cant really count because its questionable reality and also a metaphor and the standing implication is that participants dont consciously choose their revue outfits in the first place it just is)

as for why koharu can wear piercings in school outside of a costume we can only assume either that since seiran jerseys are for theater hours therefore koharu has loopholed (its during theater practice and thus IS a costume) the rules, or yakumo sensei (being yakumo kyoko) just decided koharu is allowed little a delinquency as a treat