At the heart of the criticism is the CDC’s decision to tie reopening decisions to how severely the virus is spreading in the surrounding county. The guidance says schools can fully reopen for in-person learning only in counties with low or moderate levels of transmission, which means fewer than 50 new cases per 100,000 residents over seven days or a test-positivity rate lower than 8%. Schools in counties that don’t meet that threshold should shift to hybrid learning, when students spend just some time in the classroom, with the priority on getting elementary students into the classroom, the guidance says.

Based on those measures, though, the overwhelming majority of schools in the U.S. should not bring students into the classroom five days a week. CDC Director Dr. Rochelle Walensky acknowledged in a call with reporters Friday that more than 90% of K-12 schools in the country are currently in areas of high transmission.