While everyone's making some unsettling theories, I just want to share some "interesting" theories that randomly pop up in my head:
First, Haimiya's absence from school might indicate that she actually had a reflection on Kurose's efforts to "become someone". That she, "indeed", is actually doing a similar thing, but in an exact opposite way (i.e. pretending to be someone). Her cool and strong sense of self, like what Kurose said, might actually be her way of hiding her fragile personality or her own past. She kept her composure around Kurose when they were on the beach, and by the time they went home, she finally broke down which is why she might not be coming for school for a while. She may choose to open it up at that time to Kurose, but I think she might actually not be confident enough to do so.
Another theory is that Haimiya and Aya-Sensei might actually be related. Because of their closeness, I had this strong sense that they might actually be related, could be Mother and Daughter. But they are hiding their real connection for some reason and living apart, given the fact that Haimiya is living alone in an apartment if I'm not mistaken. It's unclear whether "Aya" is Sensei's surname or given name, but it is clear that her full name is unknown. This might also an answer to why Haimiya only told Sensei and not everyone else (even Kurose) about her absence from school.
These are just my theories so I might be wrong. So let's just look forward to what will happen next in the story.
PS: I don't know why these pop up on my head, but it's probably because I'm sick of those "Terminal Illness" theories which made me sleepless. LOL