Yeogo gwae-dam 4: Moksori

While training after hours in her high-school, the aspirant singer Park Young-Eon is mysteriously killed and her body vanishes. Her ghost is invisible and trapped in the school, but her best friend Kang Sun-min, who broadcast in the lunchtime in school, is able to hear her voice. After the suicide of their music teacher, Sun-min, aided by her mate Cho-Ah, finds that another student, Hyo-Jung, died in the same elevator trunk some time ago. (imdb)