Ender's Game by Orson Scott Card
It's an easy read, and very good.
http://en.wikipedia.org/wiki/Ender's_Game
Andrew "Ender" Wiggin is a brilliant six-year-old boy, born a third child into a United States that only permits families two children each. His very existence requires permission from the government. He has been closely watched for years through a monitor on the back of his neck. The book opens with its removal. As a Third and as an exceptionally smart student, he invites scorn from the children surrounding him. At the end of the school day, the bullies of the school begin teasing and mildly attacking him for the loss of his monitor. In response and to defend himself, Ender beats the ringleader, Stilson, until he could no longer get up. He then continues to brutally assault Stilson, kicking him in the ribs as hard as he can, to end not just that fight, but all future fights with the bully and his gang. Unknown to Ender, Stilson dies as a result of the beating.
When Ender returns to his home, the reader meets Ender's older brother Peter, a boy just as brilliant but far more sadistic than Ender, and Ender's older sister Valentine. Valentine is as intelligent as Peter, but her sweet temperament is completely opposite that of Peter. The next morning, members of the International Fleet, or IF, arrive at Ender's home to ask him why he attacked Stilson. By responding that he did so to prevent Stilson from ever approaching him again, instead of saying that he attacked him for revenge, Ender passes the last test. The IF offers him a place at the Battle School, a space station where children are trained for military command. Colonel Hyrum Graff, the leader of the Battle School, believes that Ender is the last hope for the survival of mankind against the alien Buggers. He reluctantly accepts.