Detention (2011)

A serial killer known as Cinderhella is on the loose and preying on Grizzly Lake High's student body. Viewers are introduced to misfit students Riley, Ione, Toshiba, Mimi, and Clapton, as well as Sander who is infatuated with Riley. The school's principal, Verge, places them all in detention until 10 PM on the night of prom, certain that one of them is the killer. While in detention they discover that the school's stuffed bear mascot is actually a time machine and that the world is supposed to end in 1992 as a result of Ione's mother Sloane refusing to go to prom with Verge. It is also revealed that "Ione" is actually Sloane, who managed to swap bodies with her daughter and that the real Ione is in Sloane's body in 1992. Via her daughter's body Sloane stole Riley's crush Clapton, causing strife in their friendship.

They use the bear to time travel to 1992, where Ione tells them that she wants to remain in 1992, as she has become very popular due to knowing all of the trends. They also learn that Sander is the true killer and that Verge destroyed the world with his help. Although reluctant, Ione agrees to go to the prom with Verge, who agrees not to destroy the world. Riley also takes advantage of the time travel to keep her father from becoming an alcoholic.

After they return they discover that while the time travel did have some changes, the murders still occurred. Riley and Clapton, who had become a couple after the revelations of Ione/Sloan, are attacked by a jealous Sander. The three battle and Sander is killed after he is impaled on the bear mascot's fangs.

The film wraps up by showing that several of the students, two of which are Riley and Clapton, are now dating. Ione, who stayed in her mother's body, married Verge after she realized they were compatible and frequently grounds Sloan. The world is then invaded by plant based aliens, upset that humans have eaten and destroyed plants.