Criminal activity is seasonal, peaking in the summer and declining through the winter. We provide the first evidence that arrests of children and reported crimes involving children follow a different pattern: peaking during the school year and declining in the summer. We use a regression discontinuity design surrounding the exact start and end dates of the school year to show that this pattern is caused by school: children aged 10–17 are roughly 50% more likely to be involved in a reported crime during the beginning of the school year relative to the weeks before school begins. This sharp increase is driven by student-on-student crimes occurring in school and during school hours. We use the timing of these patterns and a seasonal adjustment to argue that school increases reported crime rates (and arrests) involving 10–17-year-old offenders by 47% (41%) annually relative to a counterfactual where crime rates follow typical seasonal patterns. School exacerbates preexisting sex-based and race-based inequality in reported crime and arrest rates, increasing both the Black-white and male-female gap in reported juvenile crime and arrest rates by more than 40%.