Death Bell 2 (2010)

This film is about a group of high school students and teachers who get locked in the school after the swimming instructor is murdered. In South Korea, the high school student and swimmer Jeong Tae Yeon is found dead in the pool, apparently a suicide. Two years later, teacher Park Eun Soo joins the high school, where Tae-yeon's stepsister Lee Se Hee is haunted by nightmarish visions and is bullied by the student Eom Ji Yoon. Eun Soo finds it difficult to get respect in the classroom and is backed up by an older teacher, Cha. Se Hee and her classmates are selected for an elite "study camp" held at the school during the summer break where 30 students study for their university entrance exams. The school's swimming trainer is murdered in the showers, and the words "When an innocent mother is killed, what son would not avenge her death?" found scrawled on a blackboard. A voice warns the students that they'll all be killed unless they can answer who is the murderer and why. The students and teachers find they're locked in the school when more deaths begin to happen.