Oh, their aching necks, shoulders, rib cages and lower backs. As students haul themselves and their start-of-school supplies into classrooms this week, some may be risking injury. Packed too heavy or worn improperly, backpacks can cause pain that endures into adulthood. More than 2,000 backpack-related injuries were treated at doctor’s offices, clinics and emergency rooms in 2007, according to the U.S. Consumer Product Safety Commission.