Jeanine SantucciGrace Hauck

USA TODAY

Schools listed in the company's tracker have closed for anywhere from one day to several weeks. Most temporarily moved to remote learning. Others temporarily closed with no instruction. And a small number delayed the start of school or shifted into hybrid learning, according to Burbio.

The rising number of closures comes amid a battle over mask mandates in schools and a surge in pediatric COVID-19 cases largely because of the highly contagious delta variant.

Late last month, President Joe Biden's administration announced it is investigating five states that are banning districts from mandating masks, on the grounds that such policies violate the civil rights of children with disabilities and underlying health conditions. 

"The department will fight to protect every student’s right to access in-person learning safely and the rights of local educators to put in place policies that allow all students to return to the classroom full-time in-person safely this fall," Education Secretary Miguel Cardona said in a statement.