I think the teacher Shiba-san killed Nagi and Reiji is hallucinating again. Not sure how he ended up living on the mountain with Shino'oka-san though. But if you reread the scene again where she drives off with Nagi's brother; it sounds like she's implying a room just opened up because Nagi is dead and then she gets rid of the murder weapon that she honestly could've just left there otherwise. Just my theory.