School districts across two Southeastern Pennsylvania counties are considering their broadest expansion of in-person instruction since the pandemic began after health officials moved to relax social distancing standards for school buildings.

The guidance, expected to be issued this week by the Chester County Health Department, would cut from six feet to three feet the recommended minimum social distance among students as long as their communities are below the highest levels of virus transmission.

The directive could spur nearly 30 public school districts in Chester and Delaware Counties — which together serve tens of thousands of students — to begin welcoming back more students to their classrooms. It comes as the region, and the country, grapple with how to reopen schools safely — a fraught debate that has been complicated by a slow rollout of the coronavirus vaccine to teachers.

In a letter Friday to superintendents in the two counties, Chester County Health Director Jeanne Franklin acknowledged the “urgent need to reopen schools” and called in-school instruction “vital” for all children. “Because of the layered mitigation measures you have successfully implemented,” she wrote, “we support schools increasing more in-person learning through reduced physical distancing.”

Some counties, districts and private schools have already taken the same step, notably Bucks County, where the Health Department since last year has endorsed a three-foot minimum for spacing in schools. In some other districts — the largest being Philadelphia, where the district is in a standoff with its teachers union over the safety of returning to classrooms — schools have yet to reopen in person at all.

Chester County’s announcement drew immediate pushback from the state’s largest teachers union, which said it would “inevitably lead to overcrowded classrooms” and increase the risk to students, staff, and communities.

“Six feet of social distancing is the norm in retail establishments, restaurants, and other businesses,” said Chris Lilienthal, a spokesperson for the Pennsylvania State Education Association. “It’s completely unacceptable that these rules won’t apply to Chester and Delaware schools, where students can now sit at desks within three feet of each other for up to an hour at a time.”

Many school leaders say they can’t reopen classrooms more fully while maintaining the six-foot requirement, given limited spacing and staffing levels in area schools. Some welcomed the new guidance.

» READ MORE: Cheltenham High didn’t have enough teachers to reopen in person. Other schools are facing challenges, too.

“Without the easing of that distancing for students ... it would have been impossible for us to do this,” said Marc Bertrando, superintendent of the Garnet Valley School District, which is planning next month to increase in-person instruction from two to four days a week for first through fifth graders.

West Chester Area Superintendent Jim Scanlon told families that administrators would present a plan during Monday’s school board meeting to offer students “additional in-person instruction at reduced physical distancing.” Tredyffrin/Easttown officials were also expected to discuss an expanded in-person instruction plan Monday night, while the Radnor School District, citing the county’s letter, said it would move forward Thursday with full-time instruction for K-2 students.

Franklin, the health director for the two counties, said schools would be “encouraged to reduce physical distancing” through updated guidance her department will release this week. The three-feet minimum applies even when community transmission is at “high” levels, although staff still must be spaced six feet from students.

That seems to depart from the recent guidance from the Centers for Disease Control and Prevention, which advised schools to space students six feet apart “to the greatest extent possible” when transmission is low or moderate, but called six feet “required” when transmission is substantial. The CDC defined the transmission levels based on the number of new cases per 100,000 people over seven days, as well as the percent of positive tests during that period.

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Currently, Chester and Delaware Counties have “very high” transmission, based on case incidence rates over the last seven days. But rates have been falling for weeks, as they have been around the region — a shift that has also prompted school leaders to take a new look at reopening plans.

“We owe it to our community to have a discussion,” said Joe Meloche, the superintendent of the Cherry Hill School District, which is considering expanding in-person instruction from two to four days a week — a change that could require a reduction in social distancing. The district is surveying parents to see how many would be interested and would accept the prospect of tighter spacing.

Such decisions are falling to local authorities. Neither Pennsylvania nor New Jersey health officials have changed their recommendation that schools maintain six feet of spacing.

» READ MORE: CDC gives new road map for schools without requiring schools to reopen

Franklin told The Inquirer on Monday that the county was not simply recommending that schools reduce spacing to three feet, but that schools “maximize space and at no time go less than three feet,” and follow other strategies to reduce risk.

The guidance is based on the experience of schools so far in reopening during the pandemic, she said.

”We know from case investigations that cases continue to mainly come from personal choices being made outside of academics,” Franklin wrote in an email. While the county has identified instances of in-school transmission, she said, “when schools successfully identify close contacts and send them home in a timely manner, the close contacts are becoming positive during their quarantine” rather than in school.

She also said the guidance would allow schools to plan now and reopen more fully once their county had reached a lower transmission level for three consecutive weeks.

A number of school leaders have pointed to Bucks County, where a number of schools have brought students back full-time. Despite that lesser spacing, county health officials there say they have not confirmed any transmission of the virus in schools.

“We’ve been watching Bucks County do this successfully, really since September,” Bertrando said. He said his district has yet to record any instances of students transmitting the virus to staff members, and noted that some schools are also rolling out coronavirus testing programs to mitigate risk.

In Garnet Valley, which is requiring staff to be tested, one support staff member has tested positive after close to 500 tests were administered over the last two weeks, Bertrando said.

Not all are following the counties’ lead. In Montgomery County, officials said they are still recommending six feet of spacing in schools.

“It is a question of risk tolerance for each community,” Val Arkoosh, chair of the county commissioners, said in an email. She said that in cases where the county had identified linked transmission of the virus in schools, “it has been when they are not following the six feet of social distancing rule.”

Arkoosh also noted that more students may have to quarantine if schools reduce spacing, because if students are seated less than six feet away from an infected student, they would be considered close contacts.

Others are wary of the shift. Michele Curay-Cramer, president of the West Chester Area Education Association and an eighth-grade English teacher, said she was “surprised” to see Chester County’s announcement “because the CDC was pretty clear with their guidelines.”

Reduced spacing is “going to be a huge concern for a lot of teachers, a lot of parents and a lot of students,” said Curay-Cramer, who expected that school districts would adopt the new county guidance. While she was glad the county still advised that staff be spaced six feet from students, she called that a “very difficult” reality to ensure in a classroom.

“We’ll do what we can as teachers to remain safe and keep the kids safe,” she said.