The Warrior Heir by Cinda Williams Chima

From the publisher (source: The Internet Book List):
Before he knew about the Roses, fourteen-year-old Jack lived an unremarkable life in the small Ohio town of Trinity. Only the medicine he has to take daily and the thick scar above his heart set him apart from the other high-schoolers. Then one day Jack skips his medicine. Suddenly, he is stronger, fiercer, and more confident than ever before. And it feels great - until he loses control of his own strength and nearly kills another player during soccer team tryouts. Soon, Jack learns the startling truth about himself: he is Weirlind - part of an underground society of magical people who live among us. At the head of this magical society sit the feuding houses of the Red Rose and the White Rose, whose power is determined by playing The Game - a magical tournament in which each house sponsors a warrior to fight to the death. The winning house rules the Weir. As if his bizarre magical heritage isn't enough, Jack finds out that hes not just another member of Weirlind - he's one of the last of the warriors at a time when both houses are scouting for a player. Jacks performance on the soccer field has alerted the entire magical community to the fact that hes in Trinity. And until one of the houses is declared Jack's official sponsor, there are no limits to what they'll do to get Jack to fight for them....

This book is part of the "The Heir Chronicles" series.
Here are some other books from this series: