On what was supposed to be the third day of his school operating in person since the pandemic hit last March, Cheltenham High School principal Renato Lajara woke at 3 a.m. and saw that nearly a quarter of his teachers would be out.

Lajara had known staffing would be a challenge: 18 of the school’s 130 teachers had taken leave rather than return when the building reopened Feb. 4. He was able to find only one long-term substitute. So when additional teachers then called out, he texted his superintendent.

“We just knew we weren’t going to have the manpower,” Lajara said.

Cheltenham High switched back to virtual instruction this month, after just two days of in-person schooling. In doing so, it solved its staffing problem: Nearly all of the teachers who had taken leave agreed to teach remotely, administrators said.

But it hasn’t figured out how it will reopen in person — a challenge playing out nationally as schools grapple not just with union negotiations, but the comfort levels and health complications of individual teachers.

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“It’s pretty much happening all across the country,” said Dan Domenech, executive director of the American Association of School Administrators. While many teachers are willing to return, “there are enough people who are concerned that it creates a problem. A lot of districts can’t staff an in-person education because they just don’t have the number of teachers who are willing to do that.”

Teachers with health conditions that put them at high risk if they contract COVID-19 can seek accommodations under the Americans with Disabilities Act, which applies to a wide array of workplaces. Employers can request medical information to substantiate the claim, and then discuss how to accommodate the employee.

To address the issue, some schools have been allowing teachers to work from home — using remote technology to instruct students in classrooms supervised by aides or other staff members.

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Some counties in Maryland and Virginia have hired hundreds of classroom “monitors” to oversee students who are being taught by teachers through computer screens. Around Philadelphia, some schools have also arranged for students in classrooms to be taught by teachers remotely.

In the Abington School District, “fewer than a dozen” of the nearly 620 teachers are instructing students remotely, said spokesperson Allie Artur, who noted that the district was required to provide accommodations under the ADA. “Existing building staff will serve as proctors in each classroom to monitor the students, provide in-class support, and operate the technology,” she said.

Of the Cheltenham district’s 392 teachers, fewer than 30 took leave before it reopened. But 18 of those were at the high school — a higher concentration than in other local districts, according to Superintendent Wagner Marseille.

Cheltenham was able to maintain in-person instruction for younger grades. But the high school posed a challenge: At the secondary level, long-term substitutes have to be certified in a subject area, Lajara said. And while substitutes were already in short supply, the pool was further drained by other districts that reopened before Cheltenham.

“When we went into that well, it was dry,” Lajara said. Some candidates didn’t even show up for interviews, he said.

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Some Cheltenham teachers had asked for the ability to teach remotely once the high school reopened.

“This isn’t what I wanted for any of us, but my doctor has been adamant that I not return to the building right now, and I need to listen to his judgment,” one teacher wrote last month in a message advising parents that she would be taking leave.

Another teacher, who asked not to be named due to the sensitive nature of the situation, said it was challenging to assess the risk of returning to the building. “It wasn’t so much what Cheltenham wasn’t doing — but what we don’t know about the virus,” said the teacher, who had taken leave.

The Cheltenham teachers’ union declined to comment, as did the Pennsylvania State Education Association.

At a recent school board meeting, some asked whether Cheltenham had done enough to increase teachers’ confidence in the reopening plan.

“If people have the image of the high school as like, Philadelphia … it’s different ballparks,” said board member Jennifer Lowman, referring to concerns over Philadelphia school building conditions. Cheltenham High had installed air purifiers in every classroom and air scrubbers in the hallways, she noted.

Marseille said some staff told him they did not support returning in person until there was a vaccine.

“There are faculty members … who have lost individuals to this pandemic,” he said. While the district has tried to put safeguards in place, “nothing is foolproof in terms of providing full comfort.”

While administrators are trying to find a solution, Marseille said in an interview there were “myriad challenges” with having aides oversee students while teachers worked remotely. Contract language governs how long aides can supervise classrooms, and the district would have to consider “additional compensation.”

“What does it really mean to be a substitute in a school setting?” he said, expressing some hesitancy about not having a teacher in the room. Yet, some parents would accept that “if it means their child can have some sense of a normal high school experience.”

Of Cheltenham High’s 1,386 students, 584 — a little over 40% — opted to return in-person. Under its hybrid model, only half were in the building on the two days it reopened.

To parents like Ellie Margolis, whose senior daughter reported just one to two students physically in school during her classes, reopening the building didn’t seem worth losing quality teachers.

“I’m not at all surprised” the school couldn’t stay open, said Margolis, whose daughter chose to stay virtual but had two teachers take leave. Both have since resumed teaching remotely. “There must be some kind of compromise.”