Ender's Game by Orson Scott Card

Description from Goodreads: "In order to develop a secure defense against a hostile alien race's next attack, government agencies breed child geniuses and train them as soldiers. A brilliant young boy, Andrew 'Ender' Wiggin is drafted to the orbiting Battle School for rigorous military training. Ender's skills make him a leader in school and respected in the Battle Room, where children play at mock battles in zero gravity. Yet growing up in an artificial community of young soldiers Ender suffers greatly from isolation, rivalry from his peers, pressure from the adult teachers, and an unsettling fear of the alien invaders. Is Ender the general Earth needs? Ender's two older siblings are every bit as unusual as he is, but in very different ways. Between the three of them lie the abilities to remake a world. If, that is, the world survives."

Check out the trailer for last summer's movie adaptation.