The Hunters (2011)

Alice and her friends are approaching the end of the school year where their dead-end lives will end and the chance of a new life will begin. Before heading off to college they spend one last day together in the woods, the one part of town that has always been off limits to them growing up. As they stumble upon what they thought was an abandoned fort only to find the walls dripping in blood and decompressing body parts lying around, they are startled to learn they are now a part of an undercover investigation. After being told to get out of the woods they realize they're trapped, for the Hunters, who call the fort home, never let anyone out alive.

Source: TMDb