To be fair to Girly in his original reaction to Kitano, she's the one who told him she didn't want to dress that way, so it kind-of made sense he'd go out of his way to downplay it, rather than add more focus to something he thought she didn't like.
Also, I wonder if the twist will be that everyone in that class (with the possible exception of girly) is actually a girl (cis or trans) and the whole "only one girl" thing was started to give them a bit of a shield against the rest of the school. That would make the principal's attempt to out one of them a bit odd though, unless he had some other plan Girly preempted.
In a similar vein, I wonder if the vial of "poison" will be another fake out and it'll just be some exfoliant or something, since chemical exfoliants are literally just dilute acid.