I thought it could be a take on schizophrenia as well. The ending is ambiguous because the other students don't seem to acknowledge Isoya's presence to confirm she's fine.
So was the whole murder her fantasy? What about Isoya's absence? What about the ghosts? (Her room had no one else in it when "Isoya" visited her.) Are only the ghosts fantasies? Did she kill other people as well if the murder was real?
Usually only the continuation of a story is left for people to imagine. In this case maybe what we read as well.
EDIT: On the other hand, something that supports the whole thing not being real from the start is how she speaks to the ghosts in class, with a complete lack of awareness of the classmates' audible reactions. This interpretation is way easier on the mind, too.
last edited at Dec 21, 2017 11:06PM