Enders Shadow by Orson Scott Card

Bean's past was a battle just to survive on the streets of Rotterdam. He was a tiny child with a mind leagues beyond anyone else's. Bean's desperate struggle, and his remarkable success, brought him to the attention of the Battle School's recruiters, those people scouring the planet for leaders, tacticians, and generals to save Earth from the threat of alien invasion. Bean was sent into orbit, to the Battle School. And there he met Ender....