ThinkActive is a system to encourage primary aged school children to reflect on their own personal activity data in the classroom. We deployed the system with a cohort of 30 school children, over a six-week period, in partnership with Newcastle United Foundation’s health and nutrition programme (Match Fit). The system utilises inexpensive activity trackers and pseudonymous avatars to promote reflection with personal data using an in-situ display within the classroom. Our design explores pseudonymity as an approach to managing privacy and personal data within a public setting. Within this research we explore the motivations, challenges, and opportunities for students, teachers, and third-party providers to engage in the collection and sharing of activity data with primary school children.