Schools across Pennsylvania and around the country are beginning the 2020-2021 school year while COVID-19 remains a threat in widely varying degrees, in different communities, and at different times. Every school that is opening its building to students--part-time or full-time, starting now or later in the school year--needs to contemplate the possibility that a student or staff member will become infected with COVID-19. Previous work (Gill et al., 2020) suggests that hybrid operating approaches, with part-time attendance in the school building, can substantially slow the spread of infections, but schools still need to have a plan for what to do when someone is infected. The Pennsylvania Department of Education (PDE) asked Mathematica to extend this previous work by simulating COVID-19 spread in schools under a range of different scenarios that vary based on community infection rate, grade level, operating strategy, local COVID-19 testing capacity, and the school's response to a confirmed infection. This report provides the findings from those simulations. The main body of this report presents results that describe general patterns related to school level, community infection rate, operating strategy, local COVID-19 testing capacity, and school policies for responding to detected infections. Because the research examined such a wide range of circumstances, the results of the simulations should be relevant well beyond Pennsylvania. Educators and policymakers elsewhere across the country can use these results to inform their own decisions about operating schools and setting policies for quarantining and/or closure in response to detecting COVID-19 cases among students or staff. [For the previous work, "Considerations for Reopening Pennsylvania Schools," see ED605937.]