School Sleepover Gone Wrong

Your class in high school is having a school sleepover. You and all the others thought it was going to be a normal sleepover. However, the next day, when you and your classmates finish your sleepover, and are packing up to go back to your homes, you discover that all the doors and windows leading to outside are locked. When one of your classmates go down to one of the arts rooms, they discover something horrifying.