Sounds like Kurisaki blames Haijima-sensei for whatever happened that made her senpai quit school—and her senpai probably had a crush on the teacher, or at least a soft spot for her.
Kurisaki always assumed that every girl who went to the infirmary liked Haijima, probably because she actually had someone like that around her before.
Honestly, I don’t think anyone expected the story to take such a dangerous turn. Someone really did get hurt, not just because of some funny misunderstanding.