The Forgotten Ones (2006)

All the teachers are dead. 80 high school students have been trapped for months in CornerBrook High after the world has been invaded by alien insects whose spaceships have blotted out the sun. Factions have formed inside the school, and the food supply is getting low. A group of ragtag students try and find a weakness in the man-eating insects before the "Lord of the Flies" situation inside explodes. In doing so, they just might save the world.