Part Saw, part Battle Royale (according to IMDB). Slick and quick paced, not enough time to get bored. Students selected for a special "smarter than the rest of you" class are forced to solve difficult problems in order to spare fellow students/friends from being murdered by an unknown.
One character's presence remains a mystery, but for an opportunity to provide creepy mood, and suspicion. I give the movie five stars because it's an awesome fun time and there's an intentionally hilarious scene as the credits roll.
FIVE STARS FOR AWESOME.