An increasing body of literature shows the impact of the COVID-19 pandemic and associated lockdown measures on the mental health and well-being of adolescents. However, few studies have examined how national-level lockdown measures, specifically school closures, are related to levels of peer violence among adolescents. The Health Behaviour in School-aged Children (HBSC) study allows a unique opportunity to examine cross-nationally across representative samples of adolescents, the relationship between days of school closure and levels of school and cyberbullying perpetration and victimization and physical fighting. The current cross-sectional study, which includes data from 244,405 children (50.6% female) from 42 countries and regions, collected following the return to school (August 2021 to June 2023), showed a negative association between days of school closure and all four forms of bullying perpetration and victimization, but no such relationship with physical fighting. The associations held particularly for boys and older adolescents, for whom the prevalence of bullying perpetration was higher. Results strengthen an ecological model whereby distal structural factors, such as national school policy, shape the expression of peer violence among young people. They also support a theoretical perspective in which school bullying and cyberbullying share common drivers.