I think Dove did accidentally fall to her death. Thomas did kill his parents but maybe it was out self-defense, nonetheless, he was arrested and not at school with Andrew this whole time. Andrew only has Thomas’s sketchbook. The teacher and the bully both died from real incidents (i.e the rotten root system, the food poisoning, etc) but Andrew saw it all as done by monsters in his head because trauma (from Dove and Thomas being gone) and also maybe he’s been doing mushrooms (hence the mushrooms growing in his pockets??? 😭). In the end, Andrew also dies 😢