@Malingsendal
The boy think the reason he can see the girl who died is because she have unfinished business and he thought the girl was murdered by their teacher whom she have affair to.
The boy found knife in art room and tell authority about it but they say knife is a common thing to be found in art room.
Boy run away and remember that he kill the girl because of jealousy. Since the girl is kind to him the boy think she is a pure girl. But then he found about her affair with art teacher he went mad and kill her. After that he tried to blame the murder to art teacher by placing the knife in art room.
