The Warrior Heir by Cinda Williams Chima

"Before he knew about the Roses, 16-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."