Part of the CDC's schools guidance uses indicators and thresholds for community transmission to divide counties into 4 zones: Blue, Yellow, Orange and Red. Just 2 counties (0.06%) are in the "blue" zone.

Yellow - 148 (4.7%)
Orange - 391 (12.5%)
Red - 2599 (82.8%)

According to the guidance, the thresholds are calculated based on total new cases per 100,000 people in the last 7 days and viral test positivity in the last 7 days. If the two indicators are different, the CDC says to use the higher threshold.
https://www.cdc.gov/coronavirus/2019-ncov/downloads/community/schools-childcare/K-12-Operational-Strategy-2021-2-12.pdf

Here are the recommendations, divided by whether schools implement expanded screening testing. In high transmission areas (red), for schools without testing, middle/high schools should be "in virtual only instruction unless they can strictly implement all mitigation strategies."