Introduction: It has been hypothesized that high intensity skeletal loading during growth is an effective strategy to maximize bone accrual and reduce fracture risk in old age. The POWER PE study was an eight-month, randomized, controlled, school-based exercise intervention designed to apply known principles of effective bone loading to practical opportunities to improve life long musculoskeletal outcomes. Materials and Methods: A total of 99 adolescents (46 boys, 53 girls) with a mean age of 13.8 ? 0.4 years (peri-post pubertal) volunteered to participate. Intervention subjects performed ten minutes of jumping activity in place of regular physical education (PE) warm up. Control subjects performed usual PE warm-up activities. Bone mass (DXA and QUS) was assessed at baseline and follow-up along with anthropometry, maturity, muscle power, and estimates of physical activity and dietary calcium. Geometric properties (such as FN moment of inertia) were calculated from DXA measures. Results: Boys in the intervention group experienced improvements in calcaneal BUA (+5.0%), and fat mass (-10.5%), while controls did not (+1.4%, and -0.8% respectively). Girls in the intervention group improved FN BMC (+13.9%) and LS BMAD (+5.2%), more than controls (+4.9% and +1.5% respectively). Between group comparisons of change revealed intervention effects only for WB BMC (+10.6% vs +6.3%) for boys. Boys in the intervention group gained more lean tissue mass, TR BMC, LS BMC, and WB BMC and lost more fat mass than girls in the intervention group (p < 0.05). Conclusions: Ten minutes of jumping activity twice a week for eight months during adolescence appears to improve bone accrual in a sex-specific manner. Boys increased whole body bone mass and BUA, and reduced fat mass, while girls improved bone mass at the hip and spine.