Background: School shooting events and lockdowns have increased in the United States, raising concerns about their impact on youth mental health. Method: This study assessed the association between school lockdowns and changes in youth mental health in 10,049 children who participated in the Adolescent Brain and Cognitive Development study. The exposure was school lockdowns related to violence and shooting incidents, and the outcomes were Child Behavior Checklist scores on five mental health disorders. Results: Approximately 20.3% of children reported ever experiencing a school lockdown with 39.3% of these lockdowns occurring in the past year. Past-year lockdown exposure (versus no lockdown exposure) was associated with increased symptoms of anxiety disorders ([beta] = 0.20, 95% CI: 0.01, 0.38), somatic disorders ([beta] = 0.23, 95% CI: 0.09, 0.38), and stress problems ([beta] = 0.30, 95% CI: 0.05, 0.56). Among children who reported a past-year lockdown, those indicating clinical ranges of ADHD (OR = 2.23, 95% CI: 1.17, 4.25) or stress problems (OR = 2.75, 95% CI: 1.05, 7.25) had higher odds than their counterparts of reporting that the lockdown had a lot/some impact (versus little/no impact). Implications for School Health Policy, Practice, and Equity: Emergency preparedness in schools should prioritize mental health support post-lockdowns. Conclusions: Research and funding are needed to understand the nature, frequency, and consequences of school lockdowns.