Stealing Time — Stephanie Rabig

Years after a comet hits the world of Veyrdel, the food supply is dangerously low. The Niveti, who are carnivores, finally stoop to killing Veyrdel's other race. After his classroom is attacked, Rohan gets up the courage to go into his father's time-shifter to try and prevent the violence. But a miscalculation sends him back much further than he intended, and he gets the chance to change not only his future, but everyone's.