Tonton Film Diary of June (Yu-wol-ui il-gi) (2005)

Two teenagers joining the same class in a high school die on the same day. The one to be top of the class is killed by a big knife on the street, the other - the second best of the class indeed - seems to be jumped from the top of the school building. Everybody believes that he first killed his classmate and than himself. But when the police finds small pill boxes into the stomachs of both kids confessing their murder two South Korean cops are torn into a new case of murder series. While they try to hunt down the serial killer, more pupils are getting killed...