Watched this in film studies today and its the most fucked up yet immense film I've ever seen. Basically its a Japanese film in which 42 schoolkids are put on an evacuated island with collars filled with explosives round their necks which can be detonated at any time. They then have three days to run around killing one another with provided weapons. If there isn't one winner at the end of the third day, all the collars are detonated. Boom, everyone dies. The island's also divided into areas, if you're in that zone when you're told its become unsafe, get out sharpish unless your collar goes off.
Really good (if insane) film, highly recommend it.