Sth's bad bound to happen. If the teacher really is bad, then my assumption is... itsuki's trying to protect his precious "older brother" altho he seems cunning himself. The story seems a bit all over the place with back and forth storyline, but can still be followed. Its better to reread to understand each intention of the charas, esp. Miki's twin (itsuki) and takagi. Ch 4
Shijima no Tsuki