Background: We attempted to determine whether there was a socioeconomic gradient in 9- to 11-year-old Australian children's moderate-to-vigorous physical activity (MVPA), and whether school facilities or policies supporting physical activity were associated with school-level socioeconomic status (SES) and MVPA. Methods: Children (N = 528) from 26 randomly selected schools participated in the International Study of Childhood Obesity, Lifestyle and the Environment. School-level SES was determined by the Index of Community Socio-Educational Advantage. MVPA was determined from 7-day, 24-hour accelerometry. School facilities (21 items) were evaluated with an objective school ground audit. School policies related to physical activity were collected (18 items) in a school administrator survey. Relationships among SES, MVPA, school facilities, and policies were examined using bivariate regression, correlation analyses, and analysis of variance. Results: There was a clear SES gradient in daily and in-school MVPA. School facilities or physical activity policies were not associated with SES or in-school MVPA, with the exception of presence of a sports field which was associated with lower SES schools (p = 0.02) and lower in-school MVPA (p = 0.001). Conclusions: School-built, policy, and resource environments are similar across different SES-level schools. Therefore, some other mechanism must be underlying the SES gradients seen in MVPA participation in Australian children.