CDC Director Dr. Rochelle Walensky said Friday that recently reopened schools that are already struggling with large-scale Covid outbreaks generally aren’t following federal guidelines that recommend students and staff wear masks indoors and get vaccinated when eligible.

Pediatric Covid hospitalizations in the U.S. reached their highest reported levels this week, and Walensky said school closures will occur in districts with insufficient safeguards to block community spread of the virus. Speaking at a White House Covid-19 briefing, Walensky called on schools to adopt a multi-layered approach that also includes social distancing, enhanced ventilation and Covid testing to prevent outbreaks in schools.

“I want to strongly appeal to those districts who have not implemented prevention strategies and encourage them to do the right thing to protect the children under their care,” Walensky said. “In our outbreak investigations, large-scale quarantines, or large number of cases are generally occurring in schools because schools are not following our guidance.”

Walensky, who didn’t call out any school districts by name, said the Centers for Disease Control and Prevention studied an outbreak in Northern California where an unvaccinated, symptomatic teacher read to a class without a mask, spreading the virus to students, staff and their families. And in Florida, Hillsborough County Public Schools reported this month that nearly 10,400 students and almost 340 staff were in isolation or quarantine after close contact with a positive Covid case.

The CDC also studied Covid measures in Los Angeles County, where prevention strategies kept the rate of cases in schools lower than the rate of cases in the county. Case rates among children and adolescents in schools with Covid mitigation protocols were more than three times lower last winter than community case rates, Walensky said.

Walensky added that the CDC has seen increased pediatric Covid cases and hospitalizations in recent weeks, driven by the increased transmissibility of the delta variant.

“Schools should implement as many of these prevention layers as possible simultaneously, and this serves to protect our children, even if there are inevitable breaches in any single layer,” Walensky said.