Social connectedness at school is crucial to children's development, yet very little is known about the way it has been affected by school closures during COVID-19 pandemic. We compared pre-post lockdown levels of social connectedness at a school playground in forty-three primary school-aged children, using wearable sensors, observations, peer nominations and self-reports. Upon school reopening, findings from sensors and peer nominations indicated increases in children's interaction time, network diversity and network centrality. Group observations indicated a decrease in no-play social interactions and an increase in children's involvement in social play. Explorative analyses did not reveal relations between changes in peer connectedness and pre-lockdown levels of peer connectedness or social contact during the lockdown period. Findings pointed at the role of recess in contributing to children's social well-being and the importance of attending to their social needs upon reopening. •This case-study is the first to compare social connectedness at the schoolyard from before to after COVID-19 lockdown•Sensors, observations and nominations indicated increases in interaction time, social play, network diversity and closeness.•Changes in peer connectedness were not related to pre/during-lockdown connectivity, yet this issue merits further research.•School scheduling should take into account children's social needs upon reopening.