Among the Impostors (Shadow Children)

Margaret Peterson Haddix.

Luke Garner is an illegal third child. All his life has been spent in hiding. Now, for the first time, Luke is living among others. He has assumed a deceased boy's identity and is attending Hendricks School for Boys, a windowless building with cruel classmates and oblivious teachers. Luke knows he has to blend in, but he lives in constant fear that his behavior will betray him.