Many of the teachers who were called back to classrooms Monday in Philadelphia decided instead to teach their virtual classes outside in 19-degree weather to raise awareness about what they believe to be unsafe teaching conditions. In San Francisco, after months of negotiation and after the mayor sued her city’s school system, the teachers union agreed to a deal that will set the stage for them to return to classrooms.

And in Chicago, the mayor on Sunday announced a tentative deal with the city’s teachers union that would have broken a standoff over reopening — only for the union to dispute that an agreement had been reached.

“We do not yet have an agreement with Chicago Public Schools,” the account for the Chicago Teachers Union tweeted.

The clashes come as school officials and teachers unions have struggled for months to reach agreements over when and how teachers would return to classrooms, with unions often insisting that school districts are not doing enough to safeguard the health of educators and students.

Now, President Biden has pledged to have the Centers for Disease Control and Prevention release guidelines to give school systems “minimum requirements” to ensure the safety of students, teachers and other school staff who head back to classrooms. Biden has said he wants a majority of K-8 schools to reopen for in-person instruction within 100 days of his taking office, calling school closures “a national emergency.”

“I think it’s time for schools to reopen safely,” Biden told “CBS Evening News with Norah O’Donnell” on Sunday. “You have to have fewer people in the classroom. You have to have ventilation systems that have been reworked.”

The guidelines cannot come soon enough for hundreds of thousands of families living with constant uncertainty and conflicting messages about when their schools will reopen. Proponents of reopening schools point to studies that show little spread occurring within schools, and also point to growing fears about widening achievement gaps. But opponents in many places say their school buildings are not well-ventilated enough for it to be safe. In some cases, the fight over reopening has reignited long-standing concerns about building safety.

In D.C., students finally returned to the classroom last week after a drawn-out fight with the teachers union, which raised concerns about HVAC systems and said infection rates were too high to ensure a safe return. And many other school districts have seen teacher sickouts and other protests as they continue to demand safer classrooms. In Montclair, N.J., the schools superintendent has sued the local teachers union to force the return of teachers to classrooms. The union has said it does not want educators in classrooms until they can be vaccinated and also cited facility problems, according to NorthJersey.com.

Late Sunday, after the Philadelphia Federation of Teachers announced it was instructing its members to not return to school buildings, Mayor Jim Kenney intervened to stop the school district from disciplining teachers who decided to continue working remotely. The union has been pressing for more assurances that the classrooms are well-ventilated. An investigative series by the Philadelphia Inquirer and the Daily News in recent years documented extensive issues with asbestos and lead-tainted paint in school buildings.

During a chilly outdoor news conference Monday, Philadelphia Federation of Teachers President Jerry T. Jordan said that the union still has concerns about ventilation in classrooms — and that the district’s solution to stagnant air was installing window fans.

“The district has an absolutely abysmal track record when it comes to addressing health and safety,” Jordan said. “Amidst a global pandemic, the likes of which none of us has ever seen, the school district is hellbent on forcing thousands of educators into unsafe school buildings held together, in some places quite literally, by duct tape.”

In a statement Monday, Philadelphia Superintendent William R. Hite Jr. said the district has spent $4 million assessing and upgrading school ventilation systems, and $250 million to remove asbestos, inspect buildings for lead hazards, and modernize classrooms and bathrooms, among other improvements.

“The science from the CDC is clear that when proper safety precautions are in place, the risk of transmission is low,” Hite said. “All of these layers of safety are in place.”

In Chicago, the nation’s third-largest school system and the union have been battling over a reopening plan for months. Mayor Lori Lightfoot announced Sunday that the city and the union had come to a “tentative agreement" that would have teachers return to school buildings beginning Thursday. The plan also includes measures to expedite vaccines for school workers and strict metrics on when schools or classrooms would close. But union officials are set to decide Monday whether to allow rank-and-file teachers to vote on the agreement — or whether to reject it, a move that could lead to further delays, more lockouts and would raise the potential for a strike.

Lightfoot said during a news conference Sunday that she was pressing ahead with the plan.

“Time is running out. We need our kids back in school," Lightfoot said. “We are failing children by not giving them the option to return to school.”

The overwhelming majority of Chicago’s 350,000 public school students have not seen the inside of a classroom in nearly a year.

The teachers union and the mayor have a history of disagreements. In 2019, not long after Lightfoot took office, teachers went on strike to demand more school nurses and social workers, along with funds to reduce class sizes. At the end of the impasse, which lasted 11 school days, the union declined to hold a joint news conference with Lightfoot.

The pandemic surfaced these tensions once again. Throughout the fall, while coronavirus ravaged Chicago, the city and the teachers union could not come to an agreement over when and how to reopen the city’s public schools.

Then the city declared it was moving ahead to reopen schools without the consent of the teachers union. Chicago attempted to reopen school buildings at the start of the year, giving prekindergarten students and some students in special education classes the option of face-to-face learning.

But more than 140 teachers and aides never showed up to school buildings, according to Chicago Public Schools. Some attempted to continue teaching virtually given that many of their students had opted to stay home anyhow. The school system declared them AWOL, began docking their pay and locked them out of their virtual classrooms. Even as the rest of the school system shifted back to remote learning this week, 31 teachers and 17 staff members remained locked out.

K-8 students were slated to return last week, but the city pushed the date back repeatedly as it faced an impasse with the teachers union, and feared opening without enough staff.

Dawn Reiss contributed to this report.