Enter 15-year-old Kalin March, the new kid in town, a wispy transplant from back east. His preternatural gift with horses impresses even the jocks who haze him. His single mom, Allison, juggles three jobs, and Kalin wears the same weathered Top-Siders to school each day. He’s a loner until he befriends Tom Gatestone, a jokey, popular junior whose mouthy younger sister, Landry, can wrangle a bronco with the best of the guys. (Landry, who dominates entire sections, bears more than a passing resemblance to Mattie Ross in “True Grit.”)