We have a limited understanding of the socioenvironmental factors associated with participation in physical activity among school-aged children with autism spectrum disorder (ASD), particularly regarding how the school environment may influence their participation. Using the International Classification of Functioning, Disability and Health (ICF) as a framework, this study examined the effect of body functions and structure, activity, and personal factors on in-school physical activity; and whether in-school physical activity, considered a socioenvironmental factor, is associated with out-of-school physical activity (i.e., participation) among elementary school-aged children (6–13 years of age) with ASD. Parents of 202 children with ASD (78.2% boys; Mage = 9.4 years) completed an online survey, as part of a larger study, to assess their child’s functioning and physical activity in- and out-of-school. Results indicated that the majority of children (85.1%) did not meet physical activity guidelines. In-school physical activities significantly predicted out-of-school physical activities including leisure-time moderate-to-vigorous physical activity (R2 = 0.27, F(10,154) = 5.67, p < 0.001) and meeting the physical activity guidelines (R2 = 0.23, Χ2 (10) = 31.9, p < 0.001). These findings underscore the importance of supporting children with ASD to be physically active in school, which may impact physical activity levels out-of-school.