The Montgomery County Board of Health on Friday ordered all K-12 schools to close for two weeks beginning Nov. 23, a mandate county officials described as a necessary attempt to help slow the surge in coronavirus infections.

Despite outcry from parents, the five-member board voted unanimously to impose the order, which directs schools to revert to virtual instruction. Members said that while they had heard parents' frustration, the shutdown was a proactive approach to a rapidly escalating problem.

“If we don’t do this, we will be in a significantly worse situation post-Thanksgiving holiday,” said Barbara Wadsworth, a board member who is senior vice president of patient services and chief nursing officer at Main Line Health.

The order applies to both public and private schools, along with school sports and extracurricular activities.

The decision came as the number of coronavirus cases accelerates in Pennsylvania and New Jersey. The Children’s Hospital of Philadelphia PolicyLab this week called on schools around the region to return to virtual instruction, saying that, although schools haven’t appeared to be the source of most infections of children or staff, surging caseloads are increasing the risk of in-school transmission and complicating contact tracing efforts.

So far, other counties in the region have not indicated they intend to impose similar orders. A spokesperson for Chester County — where the health department also serves Delaware County — said Friday there was “no change” in the county’s guidance to schools.

Unlike in the spring, as officials scrambled to respond to the pandemic and schools were forced to close en masse, “I see more thoughtful decisions,” said Alan Fegley, superintendent of the Phoenixville Area School District in Chester County, which was sticking Friday with its hybrid in-person and virtual offering. Like most districts, Phoenixville is also offering a fully virtual option.

Fegley said the county had been providing schools detailed guidance, advising they consider not just community transmission rates but also spread of the virus within their schools.

Officials in Montgomery County said this week they are concerned about the virus’ proliferation, after the county’s 14-day average rate of positive test results topped 5% — a threshold that experts say indicates community spread.

Hospitalizations have also been rising. Val Arkoosh, a physician who is chairperson of the county commissioners, said Thursday county hospitals were treating 171 patients with the coronavirus — up 70 people from a week earlier, and up 100 from two weeks before.

Officials said they were concerned the situation would only worsen around Thanksgiving. If students, teachers, and staff return to the classroom the week after Thanksgiving, Arkoosh said Thursday, it “could trigger a very substantial outbreak within our schools.”

While the county order runs through Dec. 6, board chair Michael Laign said it was possible it could be extended — “whether it’s by the state or us, if we’re looking at the situation at the time … deciding to go beyond that.”

The board did not take public comment Friday, but after the meeting, plans of protests against the order were percolating on Facebook.

Pennsylvania has recommended that schools in counties with “substantial” levels of transmission of the virus go back to virtual instruction. Among them are Bucks, Delaware, Montgomery, and Philadelphia Counties.

But Pennsylvania Health Secretary Rachel Levine said this week that the state is not planning to order school shutdowns as it did when the pandemic emerged in March. At that time, schools in Montgomery County, the early epicenter of outbreaks in the Philadelphia region, were the first in the state to be ordered closed.

Those initial school closures had a ripple effect, prompting Philadelphia schools to close because many staff members lived in the county.

This time, however, Philadelphia schools are already closed for in-person instruction, with no set return date.

In Phoenixville, Fegley said he expected some impact on staffing as a result of the Montgomery County order, but wasn’t sure how significant it would be. Some districts have already been grappling with staffing shortages as employees are forced to quarantine because of exposure to positive cases — not necessarily in schools, but in the community.

Other schools also said they weren’t backing off in-person instruction.

While the Archdiocese of Philadelphia’s schools in Montgomery County will switch back to virtual instruction on Nov. 23, its schools in other counties will continue to offer in-person models.

In the Neshaminy School District in Bucks County, "at this time, we are not changing our plan,” said spokesperson Chris Stanley. Neshaminy students currently attend school in a mix of online and in-person classes, though the district paused its plan to return children to school buildings full time by Nov. 2.

In Delaware County, a spokesperson for the Radnor School District, where students are attending school in a hybrid in-person and virtual model, said the district was taking its cues from the Chester County Health Department and not planning to change course.

Garnet Valley, also in Delaware County, plans to remain open “unless mandated by the Health Department or the governor,” said superintendent Marc Bertrando. With the district offering both hybrid and virtual instruction, Bertrando said in some grade levels, so few students are attending school physically — at the high school, daily attendance is 117 out of 1,600 students — that buildings are already relatively empty.

“We are so depopulated right now that going virtual has sort of happened by default,” said Bertrando, who said many students have chosen virtual instruction because of involvement in extracurricular activities — thinking there was “less of a chance they’d have to quarantine.”

In other school districts, like South Jersey’s Haddonfield, large numbers of students are quarantining. Superintendent Charles Klaus told the school board Thursday night that the district has had 12 confirmed virus cases in 14 days, and that 136 high school students have been told to stay home because of close contact with someone at school who tested positive.

“I am really ready to close the high school at almost any minute,” Klaus said.

Staff writers Melanie Burney and Kristen A. Graham contributed to this article.