Lock In (2008)

They thought they were alone...they were wrong!

What promises to be a stormy night of fun, costumes, and pranks turns deadly when a group of high school seniors, who are staging an elaborate senior prank, find themselves trapped on campus, tormented by someone from the town's dark past who's returned for bloody vengeance. One by one, the students disappear until only a few remain to battle the fiendish psychopath whose identity reveals a gruesome twist that no one could have ever suspected Lock In is a perfectly terrifying homage to the gory slashers of the 80's that's sure to make you scream in delight.