When you say "fantasy", do you maybe mean Sci Fi?
If so, I'll bet money you're talking about <a href="http://www.amazon.com/Enders-Game-Ender-Book-1/dp/0812550706/sr=8-1/qid=1169156780/ref=pd_bbs_1/103-6946418-4542238?ie=UTF8&s=books">Ender's Game</a> by Orson Scott Card.