Ender's Game

Ender's Game
By Orson Scott Card

Ender Wiggins is six years old when they take him away from his family on Earth and put him in space-based Battle School.  There, he meets other gifted children like himself, all being trained to fight the "buggers"--the supposed alien enemy.  Ender quickly rises to the top, becoming the best chance Earth has in the space war against the buggers.  Meanwhile, his brother and sister on Earth, also gifted, are planing to take over the world and start an Earth-based war.
This is an intense, action-packed, but pretty serious sci fi novel about adults using children as soldiers, the threat of invasion from other worlds, and the purpose of war.  I couldn't put it down!
There are more books in the Ender series, too, but I haven't read them yet.