It could be.. I don't think it was one of the teachers, and the creepy teacher is away so I doubt he had the time to order someone to steal it, plus if it was the case that notebook would have been enough to suspend Aye or even get him kicked out, but that didn't happen. It has to be one of the students
I wonder if in the next episode Phayu is scared of the feelings he's having for Rain since he's telling him not…
I wanna know why is he pushing him away too.. Either like u said he's scared of his feelings, or he's scared he will put Rain in danger due to the nature of his work.. My other theory is he might have found out about Rain's "crush" on Ple and he got jealous (although I doubt it)