OBJECTIVE: For optimal health, physical activity should be an integral and routine part of daily life. Youth spend a significant amount of time at school yet rarely achieve the recommended 60 min of moderate and vigorous physical activity in physical education (PE) classes or recess. This study assessed the following types of school-based opportunities to improve physical activity for youth: after-school programs, before-school programs, PE classes, extended-day PE, and short physical activity breaks during the school day. METHOD: An economic analysis conducted in 2013 compared school-based approaches to increasing physical activity. Analysis factors included costs, reach, effects on physical activity gains, cost-effectiveness, and other potentially augmenting benefits. RESULTS: Two programs were significantly superior in terms of reach and cost per student: (1) extending the school day with mandatory PE participation and (2) offering short (10-minute) physical activity breaks during regular classroom hours. After-school program costs per student are high and the programs have a smaller reach, but they offer benefits (such as childcare) that may justify their higher costs. Before-school programs did not appear feasible. CONCLUSION: Incorporating short physical activity breaks into the existing school day would be a cost-effective way to increase school-based activity. This type of program is inexpensive and has broad reach. Inserting activity breaks throughout the day is appropriate, especially when youth are otherwise largely sedentary.