Background: School children are confined to and exposed to outdoor environment that happens to be at their disposal during compulsory school time. The health-promoting potential of outdoor environment, and the use of it, is therefore important. We have studied the impact of school outdoor environment in terms of playground features, space, topography and vegetation upon the patterns of moderate to vigorous physical activity (MVPA) across ages and seasons in Swedish pupils at compulsory school. Methods: Four schools in the Middle and Southern parts of Sweden, with outdoor environments differing in playground features, space, topography and vegetation were analyzed during one school year. A sample of 196 children was drawn from eligible pupils in grades 2, 5 and 8, aged 7-14 years. PA was monitored with time-stamped Actigraph accelerometers GT3X+, measuring different intensity levels during outdoor time. Maps were used to mark places where the children stayed and what they did during outdoor time. Results: Mean MVPA during outdoor stay was 39 minutes for the entire school year, time in MVPA correlated positively with outdoor time, as did MVPA with used outdoor play area (p < 0.001). Outdoor MVPA declined with age, boys accumulated more MVPA than girls at all ages (p < 0.001). Ball play areas increased MVPA in 5th graders in September and May (p < 0.001). Overall, ball play areas increased 5th graders' relative MVPA, and helped maintaining it with increasing age in boys but not in girls, whereas woodland stimulated and contributed to maintaining girls' MVPA with increasing age. Outdoor temperature significantly impacted (p < 0.01) MVPA throughout all seasons. Conclusion: We conclude that school outdoor environment design and outdoor play time impact physical activity on a daily basis and may contribute to increasing girls' physical activity and moderate the sharp decline in physical activity by age. The school outdoor environment may thus be a potential health promoter during school time.