Childhood obesity is a critical public health issue, with prevalence rates reaching nearly one in five children. Schools may be a promising public policy intervention point. The foods schools sell and the physical activity environments they foster can influence dietary behaviors and overall physical activity. Using secondary data from a nationally representative sample of children from the kindergarten class of 1998-1999 and nonexperimental methods, this study examines the associations between the food and physical activity environments in school and body mass index (BMI) for low-income boys and girls in the 8th grade during 2007. Results reveal that participating in school sports is associated with a 0.55 lower BMI score for boys. For low-income girls, eating the school breakfast is associated with a 0.70 higher BMI score and eating the school lunch is associated with a 0.65 higher BMI score. Each hour spent on homework is associated with a 0.02 higher BMI score for low-income girls. These findings suggest that schools may influence adolescent BMI and that there is room for improvement in school food and physical activity environments to promote healthier weights for low-income boys and girls.