For a comparison of the CDC categories and California’s tiers based on Blueprint for a Safer Economy, see here.
“If community transmission is high, students and staff are more likely to come to school while infectious, and COVID-19 can spread more easily in schools. The association between COVID-19 incidence and transmission in school settings and levels of community transmission underscores the importance of controlling disease spread in the community to protect teachers, staff, and students in schools. This means that all community members, students, families, teachers, and school staff should take actions to protect themselves and others where they live, work, learn, and play. In short, success in preventing the introduction and subsequent transmission of SARS-CoV-2 in schools is connected to and facilitated by preventing transmission in communities.”Transitioning from CDC’s Indicators for Dynamic School Decision-Making (released September 15, 2020) to CDC’s Operational Strategy for K-12 Schools through Phased Mitigation (released February 12, 2021) to Reduce COVID-19