Evidence suggests that young girls are less likely than boys to be physically active at school and are less responsive to physical activity interventions. This study employs a participatory case study approach to explore what young girls and their teachers want from physical activity interventions during the school day. The project aims to distance itself from a hierarchical researcher-participant dynamic and make sense of the issues through a shared conceptualisation and co-researching partnership. One class of British Year 2 girls, their class teacher and their head teacher participated in this study. The girls and their teachers designed their own physical activity intervention, implemented it, measured changes in step count (using pedometers) pre- and post-intervention and reflected on the process. Qualitative data were gathered via focus groups with the girls and semi-structured interviews with their teachers. Key themes that emerged were a desire for choice in how they were active, and an interest in working together as a team within a social framework to increase activity.