In an analysis of 520 U.S. counties, the CDC found that pediatric cases rose more sharply in places without school mask requirements. And in a separate report that looked at Arizona’s two largest counties, the agency found that schools without mask requirements were 3.5 times as likely to be forced to close as schools with them.

Though polls show a majority of parents support mask requirements — and despite recommendations from pediatricians and the CDC — schools remain bitterly divided over whether to implement them. Opponents of mask mandates say parents should get to decide whether their children wear them.

The pandemic has already taken a toll on the new school year. In a third report, the CDC said more than 900,000 students in 44 states had been affected by closures since the start of school. Many schools are contending with challenges on multiple fronts, including outbreaks that force staff to quarantine, and staff shortages that make it difficult to find replacements.