Man, being Kaoru is pain. I'm confused as a reader, no idea how would a 17-year old put in that situation do. She made some bad calls, but the angst is palpable.
I really, honestly can't get a read on what Rin's aiming for. One moment she seems serious, and another like it's just all a plaything for her to not feel lonely or bored. The fact she is loaded and doesn't really have to care about anything, while other main characters have to struggle and can't afford to make bold moves doesn't help the impression she gives either.
Still, no idea why MC, being a smart guy, didn't immediately tell authorities about what she told him. I would be meeting with the teacher, school psychologist, principal, her parents/guardian (by phone if needed), and probably police within an hour of leaving her at a hospital. She is a danger to herself and others - obviously unstable. Her blackmail holds no real power, the second he informs appropriate people of it. But I guess we wouldn't have the story if he did :shrug:
This is a really uncomfortable, yet engrossing read.