Background: Physical activity is important for children's physical, mental and cognitive wellbeing, but many children are insufficiently active. Schools offer a promising setting in which to enable positive health behaviours and teachers can play an influential role in facilitating pupils' physical activity. The aim of this study was to use pupils' physical activity data from wearables to inform teacher-led strategies to improve pupils' physical activity outcomes. Methods: Participants included 11 teachers/classes, and 228 pupils aged 8-11years from eight primary schools in England. Baseline pupil physical activity was measured for 2 weeks using a commercially available accelerometer, before data visualisations were used to inform teacher-led strategies with the aim of improving pupils' physical activity. These strategies were then implemented, with physical activity being measured post-data-sharing discussions. Mixed-methods analysis explored the feasibility of using data visualisations to inform teachers' understanding of their pupils' physical activity and the development and implementation of teacher-led, school-specific strategies to increase pupils' physical activity. Results: Teachers understood and reflected on the data visualisations and used their knowledge of the school context to develop and implement bespoke school-specific strategies across the school day which successfully increased physical activity. Strategies included altering the physical or social environment, providing more opportunities and addressing physical activity inequalities. Teachers were encouraged when data showed increases in pupils' physical activity and many expressed the desire to continue addressing physical activity, with a particular focus on the least active pupils. Conclusion: These findings suggest that data-driven insights could be used effectively to inform schoolbased, teacher-led strategies to increase physical activity.