Snow — and, in many cases, COVID-19 — snarled schools’ plans on Monday, what was to be the first day back for thousands of students after winter break.

A number of districts in Pennsylvania and New Jersey closed because of winter weather concerns. The Central Bucks School District said it shut in part because “the omicron-related spike in COVID-19 cases” created “an unprecedented need for substitute teachers that far exceeds the number available.”

“All of this really comes down to, do we have enough of our staff to run a safe and orderly school day every day? That is going to be the crux of the next two to three weeks,” said Christopher Dormer, superintendent of the Norristown Area School District, which announced Monday night that it would operate virtually for the rest of the week, due to absences among staff members and contractors.

In Philadelphia, which had previously planned for a virtual, staff-only school day on Monday, concerns about surging case counts, a lack of availability of COVID-19 tests, and worries over building conditions caused the teachers union to ask Superintendent William R. Hite Jr. for a week of virtual instruction to give the school system “time to ensure that scientifically recommended mitigation strategies are ready for implementation in every work location.”

» READ MORE: As much as 3 inches of snow in Philly, as storm picks up overnight

The Philadelphia Federation of Teachers said that more than 1,000 of its 13,000 members reported positive COVID-19 cases to the union, with almost 2,000 in its members’ households. More than 4,000 members said they needed COVID-19 tests over the winter break, with more than half reporting they had trouble finding tests, union officials said.

“We believe these numbers are likely a snapshot of the reality of the surge amongst our members, and ask that you take this seriously,” PFT president Jerry Jordan wrote in a letter sent Sunday to Hite.

Monica Lewis, a School District of Philadelphia spokesperson, said the system is keeping a close eye on case counts, but “we remain confident that we’ll be able to have in-person learning [Tuesday].”

On Monday night, the district announced that about one-third of its schools — 77 in total — would be virtual beginning Tuesday and at least through Friday, because of high numbers of expected teacher absences. Staff are still expected to report to work in person, and those who are quarantining at home but asymptomatic may be able to teach from home, after consulting with principals.

Hite, in a message to parents, said the district will continue to “make school-by-school decisions in the best interests of our students and staff.”

Students at the remainder of the system’s 216 schools are expected to report to classes in person Tuesday.

Philadelphia Health Commissioner Cheryl Bettigole reiterated her position that schools should remain open whenever possible.

“The Health Department believes that in-person learning is critical to the well-being of our children and deeply appreciate the commitment of the city schools to maintaining in-person education as consistently as possible,” Bettigole said in a statement Monday.

Some schools and districts had previously said they were moving to remote instruction for part of January to tamp down the spread of COVID-19. The Mastery Charter network in Philadelphia and Camden will operate virtually through Jan. 18; so will the Collingswood, Oaklyn, and Burlington Township school systems. Pennsauken students will learn virtually through Jan. 7.

In New Jersey, Gov. Phil Murphy said Monday that the state’s mask mandate will remain in place for the foreseeable future, but said he had no plans to order schools to return to remote learning.

“We will do everything we can to keep our students in school,” Murphy said at a news conference.

One Burlington County district, Westampton Township, was expected to consider a plan Monday night to provide additional pay — $33.60 an hour — to 18 teachers, aides, counselors, and other support staff to conduct after-hours contact tracing to assist overwhelmed school nurses.

“They just need more people and more bodies,” said Cathy McManiman, a kindergarten teacher and president of the Westampton Education Association.

Most districts on Monday assessed the impact COVID-19 cases among staff and their families would have on schools’ ability to educate students this week. A nationwide shortage of substitute teachers complicates the picture.

Haddon Township schools planned to reopen Tuesday, after a snow day, but scheduled half days for the rest of the week to minimize the risk of students removing masks to eat, said Superintendent Robert Fisicaro. Nearly 100 students and staff have tested positive since Dec. 23, compared to 127 for the entire school year prior, he said.

“This pandemic is not easy on anybody. This situation has made life more complicated across the board,” said Fisicaro.

After canceling school Monday, Lower Merion was also prepared to reopen Tuesday, though “we are not operating in ideal conditions and several realities can impact our plan at any time,” Superintendent Khalid Mumin told parents. In some cases, he said, children might be taught by teachers working remotely while another adult supervises the classroom. He advised that students pack a lunch to ease the workload on a limited cafeteria staff, and also warned of possible bus driver shortages.

Given the short supply of drivers, the Deptford Township School District canceled after-school activities and athletics for Tuesday to allow its available drivers to focus on transporting students for a full school day.

By Monday evening, Central Bucks announced that all schools would open in person Tuesday.

“As employee absentee rates continue to rise, and substitute availability rates continue to decline, the district has developed a plan to deploy additional internal resources to address unfilled positions,” Superintendent Abram Lucabaugh told families in a message. But if unfilled absences reach “a level that is unmanageable despite these additional efforts,” he said, schools may need to shift to a modified schedule. His message included links for people to sign up as substitute teachers.

A number of other districts across the region closed simply because of snow, with no mention of COVID-19 concerns.

Cherry Hill, for instance, closed because of “the uncertainty of today’s predicted inclement weather,” designating Feb. 21 as a makeup day. One of its schools, Barclay Early Childhood Education Center, will shut to all preschool students without special-education plans because of high numbers of staff needing to quarantine because of COVID-19 cases.

Before calling a snow day Monday, the Upper Darby School District had planned a virtual day for Beverly Hills Middle School, where 16 teachers were expected to be out.

Other Upper Darby schools also had significant teacher absences. But the district said later Monday that it planned to open all schools Tuesday, while noting that parents could expect some elementary classes to be combined due to lack of staff.

”It will continue to be our goal to keep schools open for in-person instruction, but we are going to need everyone to support us,” Superintendent Daniel McGarry said in a message to parents, asking families and staff to continue to screen for symptoms and communicate with school nurses and principals.

Elsewhere in Delaware County, the William Penn School District decided to operate virtually for a week.

“The increase in COVID incidences and positivity rates, combined with the limited supply of test kits make us cautious about how we move forward to support the health and safety of both students and staff,” Superintendent Eric Becoats said in a message to families over the weekend.

After calling a virtual day due to snow Monday, Dormer, the Norristown superintendent, hoped to be able to open schools in person the rest of the week. However, he said the situation could shift suddenly: Like other area districts, Norristown has struggled to hire substitute teachers this year. Support staff and contracted workers like bus drivers have also been hard to find; Dormer personally has filled in staffing lunches.

“We just don’t have as much wiggle room as we’ve had in the past to absorb a surge of absences for something related to COVID,” he said.

By Monday evening, he had called in-person school off for the rest of the week, telling families the district was short too many critical positions — and that “making a decision each night or morning would not be fair to parents who need to make child care and other arrangements.”

In Camden County, Lawnside School Superintendent Ronn Johnson said his district has had only a handful of cases among its 60 teachers and support staff, and plans to remain open.

Johnson said he worried about the impact of shifting to remote instruction for parents who need child care.

“There may come a time when we have to go remote,” Johnson said. “Now is not the time.”

Staff writers Laura McCrystal and Justine McDaniel contributed to this article.