That went to hell in a handbasket pretty quickly, damn. So, what are the chances that most of them are, in reality, already dead and are just trapped in a loop reliving these horrifying memories?
The students of varying grades in one classroom is reminiscent of Higurashi no Naku Koro ni, it's a bloody time loop horror too... also set in the countryside... and in June... and the protagonist is a city slicker who moves to the countryside...