Schools are a key setting for promoting physical activity in children. There is little evidence on the potential of widely implemented programs to improve the quality of physical education (PE). The aim was to assess the effects of a short training course for classroom teachers on the quality of PE, assessed as activity time during PE. A quasi-experimental study was conducted with 6 intervention (N = 86) and 13 control classes (N = 151). Schools were selected in a random procedure (26.9% participation). Participation in children was 86.2% (mean age 7.8 years, 48.9% girls). Physical activity was assessed objectively using accelerometers. Effect on time spent in sedentary, moderate, vigorous and moderate-to-vigorous (MVPA) activities, steps and counts/minute during PE were analysed using t-tests and mixed linear models. Physical activity time increased significantly in the intervention but not in the control group between baseline and follow-up (relative increase in moderate-to-vigorous physical activity of 12% in intervention group). Increases were strongest in girls and in children inactive at baseline. In the mixed linear models adjusted for clustering, the effects were significant in girls for vigorous activities, sedentary time and counts/minute, in inactive children for steps. Results indicate that a short training course for classroom teachers can have subtle positive effects on physical activity time during PE. Girls and the most inactive children at baseline profited most from the intervention.