The Battle School is constructed for the sole purpose of training children to fight in outer space. The children are not coddled or protected, but allowed to fight among themselves. The teachers deliberately set up the rules to pit the children against each other, in order to teach them strength of character and to see how they react.
"...you will get pushed around. And when you do, don't come crying to me. Got it? This is Battle School, not nursery school... If you don't like getting pushed around, figure out for yourself what to do about it..."
(Card, Ender's Game, Google Books)
Battle School is isolated and far from parents. The only authority figures are more interested in seeing how the children adapt than in protecting them, and so the children who don't get expelled become hard and cynical. Ender avoids cynicism for the most part, but the constant brutality makes him very pragmatic, which sometimes comes across in violence. Without the Battle School, which taught persistence through adversity through never playing fair, Ender would never have learned the lessons and the determination necessary to win the Bugger War.