School playtime (recess) provides children an opportunity to engage in a range of active and sedentary play behaviors on a daily basis. However, little data have investigated changes in playtime behaviors over time. The aim of this study was to investigate how children's physical activity levels, the size of their social group, play behaviors and social interactions changed over one academic year (Study 1), and during the transition from Year 5 (aged 9-10 years) to Year 6 (aged 10-11 years; Study 2). Primary school children were directly observed during school playtime using the System for Observing Children's Activity and Relationships during Play. Results revealed children engaged in moderate-to-vigorous physical activity for at least half of the intervals observed. Incidents of physical antisocial behavior significantly decreased, while incidents of verbal antisocial behavior and time spent alone increased across Study 1. Incidents of verbal antisocial behavior, time spent alone and engagement in playground games significantly decreased across Study 2. The data suggest that children are highly physically active during school playtime, and while their play behaviors and activity levels fluctuate, these fluctuations were generally small over one academic year and in the transition between Year 5 and Year 6.