The Centers for Disease Control and Prevention announced Friday it is dropping classroom distancing recommendations from 6 feet to 3 in most cases, a move that could accelerate the reopening of school buildings locally and nationally.

Citing evolving science and new studies, the CDC said in elementary schools where mask use is universal, children can be spaced 3 feet apart, regardless of COVID-19 community transmission rates. Middle and high school students should still remain 6 feet apart if community transmission is high.

Six feet of distance is still recommended between adults and between adults and students.

“CDC is committed to leading with science and updating our guidance as new evidence emerges,” Director Rochelle P. Walensky said in a statement. “Safe in-person instruction gives our kids access to critical social and mental health services that prepare them for the future, in addition to the education they need to succeed.”

The announcement addresses a key issue in the school reopening debate: Even as states work to vaccinate teachers, and the federal government steers billions in stimulus money to districts, many school leaders had said that maintaining 6 feet of distance has been a barrier to more fully reopening.

Leaders of teachers’ unions, meanwhile, have argued that pressure to reopen schools shouldn’t drive a change in recommended distancing, and voiced concern about the new federal guideline.

“We are concerned this change has been driven by a lack of physical space rather than the hard science on aerosol exposure and transmission,” Randi Weingarten, head of the American Federation of Teachers, said in a statement.

In endorsing a change, the CDC pointed to an increasing number of U.S. and international studies that the agency said showed low transmission in schools that used other mitigation measures, even when students were spaced less than 6 feet apart.

The update marks a shift from the agency’s guidance to schools last month, when it had described 6 feet of spacing as “required” while community transmission of the virus was substantial.

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In the Philadelphia area, health officials have taken varying approaches. Many schools have already shifted to a 3-foot minimum and have returned students full time — including in Bucks County, where health officials endorsed the lesser distance last year and have said they have not confirmed any cases of in-school transmission. The Chester County Health Department, which also serves Delaware County, followed suit last month.

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The Philadelphia School District had been holding firm on the 6-feet rule, citing CDC guidance. The city school district has reopened some elementary schools to prekindergarten through second-grade students; no return date has been set for students in third through 12th grades.

Superintendent William R. Hite Jr. said the CDC guidance shift is significant as the district plans to announce new groups of students eligible to return to buildings weekly.

“At least for the interim, we’re going to be naturally meeting with the [Philadelphia Federation of Teachers] and considering the new guidance, and then we’ll have to do an analysis of every learning space again” to determine how many children can safely fit inside rooms, Hite said. In some of the district’s old, small buildings, the guidance may not allow for many more students to return.

“For other schools, it could allow us to bring back a lot more children, particularly in those schools where there’s a lot of demand,” Hite said.

As for September, “we’re working towards bringing all children back to a full in-person experience,” the superintendent said.

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PFT President Jerry Jordan said the union would review the guidance and science.

“We share the concerns that our members have regarding changing protocol and guidance,” Jordan said. “In an already confusing and scary time, changes in requirements add another layer of stress and uncertainty. Like our members, we want school buildings to reopen when they are safe to do so.”

Upper Darby School District Superintendent Dan McGarry said the CDC shift was significant.

“Does 3 feet help? Of course. It allows us to get more kids in,” McGarry said. But the need for social distancing will continue to affect what happens in classrooms, he said, including how children play together.

“It’s still 3 feet, vs. normal school,” he said.

And other challenges involving in-person school remain.

The CDC continues to recommend that children be 6 feet apart when masks are off, including while eating, something McGarry’s district has yet to figure out.

Currently, K-5 students who have returned in person part time in Upper Darby aren’t eating breakfast or lunch while at school; instead, they get grab-and-go meals to eat at home. As the district looks to transition to a fuller day in person, it is considering how to use its available space for mealtime, which may involve buying more desks or adding plexiglass to tables.

”It’s a monster undertaking for us,” McGarry said.

In the Cheltenham School District, Superintendent Wagner Marseille said the guidance likely wouldn’t speed up reopening plans this spring in his district, where K-8 students are currently attending school in a hybrid in-person and virtual model. (The high school is still operating virtually due to lack of staff willing to teach in person, though as teachers have gotten vaccinated, Marseille expects to be able to open the high school after spring break.)

”We would really have to get a pulse of our community and staff” before reducing spacing in classrooms, Marseille said — tough to do with just two and a half months left in the school year, he said.

But “we can begin to see much more of a normal return” for this fall, he said. The district wants to get as many students as it can back in its buildings in the coming school year, he said, and “3 feet gives us the opportunity to do that.”

New Jersey Gov. Phil Murphy earlier this week pledged that the state’s schools will fully reopen this fall.

”We know there are students across our state who have fallen behind due to the burden and stress of remote learning and it is time to stem this tide before more students fall away,” Murphy said at a COVID-19 briefing. “A full year out of their classrooms is not how students move forward or how our world-class extraordinary educators move forward.”

Marie Blistan, head of the New Jersey Education Association, said it would take time to outfit older buildings with adequate ventilation, air purifiers, and other measures.

“All of those tools still need to be in place. It’s going to take a while,” Blistan said. “It’s going to be a gradual reopening.”

The CDC’s shift was good news for Cinnaminson Superintendent Stephen Cappello, who had already planned on announcing that the district will phase in about half of its grade levels for fully in-person learning beginning March 29, with the remainder returning by April 19.

“This did not change that schedule,” Cappello said. “The timing just happened to work out really well. This news is an important step forward.”

Camden Superintendent Katrina McCombs said she was “waiting with bated breath” for more information from state and county officials on what the new guidelines would mean for her district.

Camden, remote since last year, plans to return its pre-K through second graders, special education, and English language learners on April 12 and the other grades by June.

“It could mean that we are in a position to bring more students back earlier,” McCombs said. “But at the heart of our decision-making is student and staff member safety.”

Lawnside School Superintendent Ronn Johnson cautiously welcomed the new distancing guidelines. His South Jersey district, with about 300 pre-K to eighth graders, has been mostly remote since last year with only special needs students getting in-person instruction.

“You hope things are based in science and not politics,” Johnson said.

Johnson said the school board will decide if the district remains in a hybrid model beyond April 16, as currently scheduled. In January, three out of four parents planned to keep their children remote, but sentiments are changing, he said.

“I’m not going to tell Lawnside parents what to do with their children,” Johnson said. “We’re prepared if the kids come back or if they stay out for the remainder of the year.”

Staff writer Allison Steele contributed to this article.