Death Academy (2005)

Ten years ago four students were killed at a school, the killer was arrested and put in jail. Now, a couple of students have decided to do an essay on the murders. They get in touch with the killer who claims that he's innocent and that the principal is in fact the real killer. They take his word for it and decide to break into school one late night to find proof. What they don't know is that someone doesn't want the truth to come out and as the night falls, they soon realize that they're trapped inside the school with a vicious serial killer.