Among the Imposters by Margaret Peterson Haddix

Luke Garner is free. Well, sort of. He now has a fake ID, and is attending Hendricks School for Boys, but he has a rough start. Luke can never find any of his classes, the boys in his dormitory make fun of him and push him around every night, and can't seem to fit in. Then, Luke finds a door, which leads to a small scattering of trees and wilderness. Finally, Luke has found a place where he belongs. But there's something going on in those woods that Luke should be terrified of, because no third child in Hendricks School for Boys is safe, including Luke.

Among the Imposters is the sequel to Among the Hidden, and like Among the Hidden, it takes a while to get to the really exciting part. However, it's one of those books that forces the reader to keep reading, even when it's 2 o'clock in the morning.