Memento Mori (1999)

Min-Ah finds a strange diary, capable of arousing hallucinations, kept by two of her senior fellow-students, Hyo-Shin and Shi-Eun, who seem to have an unusually close bond. But Hyo-Shin suddenly kills herself, for no obvious reason, and the entire school is shocked and depressed. Min-Ah, however, starts to feel different. It's almost as if she's somehow possessed by the dead girl...