Background Large-scale school closures have been implemented worldwide to curb the spread of COVID-19. However, the impact of school closures and re-opening on epidemic dynamics remains unclear. Methods We simulated COVID-19 transmission TRANS dynamics using an individual-based stochastic model, incorporating social- contact data TRANS of school- aged TRANS children TRANS during shelter-in-place orders derived from Bay Area (California) household surveys. We simulated transmission TRANS under observed conditions and counterfactual intervention scenarios between March 17-June 1, and evaluated various fall HP 2020 K-12 reopening strategies. Findings Between March 17-June 1, assuming children TRANS <10 were half as susceptible to infection MESHD as older children TRANS and adults TRANS, we estimated school closures averted a similar number of infections MESHD (13,842 cases; 95% CI: 6,290, 23,040) as workplace closures (15,813; 95% CI: 9,963, 22,617) and social distancing measures (7,030; 95% CI: 3,118, 11,676). School closure effects were driven by high school and middle school closures. Under assumptions of moderate community transmission TRANS, we estimate that fall HP 2020 school reopenings will increase symptomatic illness among high school teachers (an additional 40.7% expected to experience symptomatic infection MESHD, 95% CI: 1.9, 61.1), middle school teachers (37.2%, 95% CI: 4.6, 58.1), and elementary school teachers (4.1%, 95% CI: -1.7, 12.0). Results are highly dependent on uncertain parameters, notably the relative susceptibility and infectiousness of children TRANS, and extent of community transmission TRANS amid re-opening. The school-based interventions needed to reduce the risk to fewer than an additional 1% of teachers infected varies by grade level. A hybrid-learning approach with halved class sizes of 10 students may be needed in high schools, while maintaining small cohorts of 20 students may be needed for elementary schools. Interpretation Multiple in-school intervention strategies and community transmission TRANS reductions, beyond the extent achieved to date, will be necessary to avoid undue excess risk associated with school reopening. Policymakers must urgently enact policies that curb community transmission TRANS and implement within-school control measures to simultaneously address the tandem health crises posed by COVID-19 and adverse child TRANS health and development consequences of long-term school closures.