Building design and grounds might contribute to physical activity, and youth spend much of their daylight hours at school. We examined the associations among school building footprints, the size of school grounds, and in-school physical activity of 1566 sixth-grade girls from medium to large middle schools enrolled in the Trial of Activity for Adolescent Girls (TAAG). The school building footprint and the number of active outdoor amenities were associated with physical activity among adolescent girls. On average, the school footprint size accounted for 4% of all light physical activity and 16% of all MET-weight moderate-to-vigorous physical activity (MW-MVPA) during school hours. Active outdoor amenities accounted for 29% of all MW-MVPA during school. School design appears to be associated with physical activity, but it is likely that programming (eg, physical education, intramurals, club sports), social factors, and school siting are more important determinants of total physical activity.