Pennsylvania’s largest teachers union on Wednesday urged K-12 schools to require masks in school buildings, a measure that state officials are encouraging but not mandating.

The Pennsylvania State Education Association cited the threat of the highly contagious delta variant of the coronavirus, which is spiking infections and hospitalizations across the state and nationally, including among children.

The U.S. Centers for Disease Control and Prevention recommends masks in schools for students, staff and teachers.

“Masking up is essential to keeping in-person learning going all year,” said Rich Askey, PSEA president.

“We are at a crossroads, and what our schools decide now will set the stage for what this school year looks like. If we’re going to be able to keep our schools open for in-person instruction all year, we need to make the right decisions now,” he said.

Masking has become a highly contentious and politicized issue, with heated discussions taking place at the local level as school boards decide what their policy will be as schools reopen for the fall. Some Pennsylvania districts said they will require masks, while others have decided to make them optional.

Joseph Roy, superintendent of Bethlehem Area School District, announced Wednesday that masks will be required in school buildings, citing the exponential spread of the virus in surrounding Northampton County since early July.

Roy said the district’s health experts have determined that the ongoing surge means “there’s no more latitude” on mandated, universal masking. The measure will be in place at least through the end of September.

“I’m not happy that that’s where we are, but I have to do my duty to protect our kids and our staff and our community and it’s clearly the right thing to do,” he said.

The state Health Department is urging school districts to follow federal guidance and require masks for all students, teachers and staff, regardless of vaccination status, but Alison Beam, the acting health secretary, said this week that a statewide mandate is not under discussion.

Statewide, confirmed infections have nearly doubled over the past two weeks to an average of 2,000 per day. Hospitalizations are up sharply and deaths have started to rise again, too.