I'm pretty sure the gag here is this is, mostly, all in the main character's head that she is doomed to be the "Loser Childhood friend." Even this chapter leans on harem tropes with the whole accidental pervert shtick, where the male lead straight up says that in reality it's super awkward for...
People getting salty at the teacher when he was absolutely right lol. There's been multiple incidents now where Arase has gotten carried away, on school grounds no less, to the point where it could have led to really bad incident. Up til now, the only person who suspected anything was Hifumi...
I'm getting the feeling that Ooki is actually an exhibitionist. She probably don't mind Ogata gawking at her boobs but then feels clamjammed by Katagiri.
@Tersione pretty sure those scars are just from the leftover side effect of her past surgery. The fact that she's wearing a shirt just means she's, understandbly, self-conscious about them.