Virginia’s largest school system is pausing plans to return thousands of children to classrooms — an announcement that came the same day Northern Virginia teachers’ unions urged Gov. Ralph Northam (D) to switch the state to online-only learning.

Fairfax County Public Schools, which serves 186,000 students, was supposed to send 6,800 pre-kindergartners, kindergartners and special education students back into school buildings on Tuesday. They would have joined the roughly 8,000 young children, special education students, and career and technical students who have already returned to classrooms.

But in a letter sent to families Monday, Superintendent Scott Brabrand said coronavirus cases in the area have exceeded “the threshold to expand our in-person learning.” Groups of students that are currently learning inside Fairfax classrooms will continue to do so, at least for now, he wrote.

“We made this decision as soon as new health metrics were released and are communicating it to you immediately as promised,” Brabrand wrote. “We always anticipated the need to potentially adjust our return to school plans as necessary during this ongoing pandemic.”

The delayed group of students will learn online only at least until Nov. 30, Brabrand wrote, at which point Fairfax officials may reevaluate. He closed the message with a warning: “These next few months will not be easy.”

Also on Monday, teachers associations representing 12,000 employees in Fairfax County, Arlington, Loudoun County, Manassas Park and Prince William County sent a letter to Northam saying that he should return Virginia to Phase 2 of the state’s reopening plan. The state is in Phase 3, which allows all students to receive “in-person instruction . . . with social distancing measures in place,” according to guidance posted online.

Under Phase 2, face-to-face teaching would be more restricted: Only special education students, English language learners, and preschoolers through third-graders would be allowed to learn inside schools. But the teachers groups want Northam, a physician, to go beyond that and request that all public schools provide online-only instruction until cases begin to trend downward.

“It continues to be clear that Northern Virginia is past the point of safe metrics for in-person learning in our school buildings,” the teachers wrote. “Everyone, including educators, wants our schools to be back to normal, but by opening when it’s not safe to do so, we increase the likelihood that normal will never come.”

Northam did not directly respond Monday to questions about whether he will grant the groups’ request or whether he had replied to their letter — although he seemed to shy away from the idea of a statewide order to close schools.

“As I have said since the beginning of this pandemic, we have enormous diversity of school districts within our Commonwealth — a one-size-fits-all solution simply does not make sense,” Northam said in a statement.

The governor added that he is collaborating with the Virginia Department of Health to supply school districts statewide with up-to-date health data. The governor said he trusts that individual school systems will make “the best decisions” to keep their students and staff members safe. And he called educators in the state “heroes [who] have gone above and beyond” during the pandemic to keep kids learning.

Parents, meanwhile, are fiercely divided on the issue. Although some applaud school officials’ caution and say they would like to keep their children home for the foreseeable future, others are demanding in-person instruction. Among other issues, they cite the academic, emotional and mental toll that online learning is taking on kids.

In response to the teachers’ letter Monday, groups of concerned parents in Fairfax, Loudoun, Arlington, Prince William and Alexandria City published a strongly worded letter of their own. They wrote that the coronavirus risks involved in in-person teaching are manageable and that the academic, mental and emotional harm done to children through online learning may last years.

The groups — OpenACPS, OpenFCPS, LCPS Can Do Better, #TeachMEinPersonPWCS and Arlington Parents for Education — claim to represent more than 5,300 parents in Northern Virginia.

“We have to learn to live in a world where covid exists and children are allowed to attend school,” their letter says. “There is no such thing as a zero-risk environment for anything.

“Leaders and elected officials should weigh the balance of risks and choose to do less harm to the constituency without a voice in this fight: children.”

Coronavirus cases are spiking nationwide — the total number passed 11 million on Sunday — and D.C., Maryland and Virginia are no exception. The three jurisdictions reported a cumulative increase of more than 3,100 new cases on Sunday: 1,840 in Maryland, 1,161 in Virginia and 163 in the District, setting a record in the city for the seven-day average for new infections.

Officials in the two states and D.C. have begun imposing tighter restrictions in a bid to reverse the trend. In Virginia, new rules will take effect at midnight Monday that decrease the number of people allowed for gatherings from 250 to 25, as well as lower the age at which people must wear a mask in public from 10 to 5.

The new limitations do not directly address schools, which have been largely shuttered statewide since March, when the coronavirus pandemic first started spreading across the United States. As cases dropped in late summer and held steady through the early fall, smaller, more rural school systems started opening back up — for example, Fauquier County Public Schools allowed 7,000 children back into classrooms last week.

Larger school districts in more urban areas took a more cautious approach. School systems in Northern Virginia — including Loudoun and Fairfax counties, which together instruct more than 250,000 children — began allowing small groups of vulnerable children back into classrooms in late October and early November.

But as cases began to rise in late fall, some of these systems started having second thoughts. In addition to Fairfax County, Arlington Public Schools — which enrolls 26,000 — also recently announced that it would delay returning lower-schoolers to buildings until 2021.

In his message to families Monday, Fairfax Superintendent Brabrand said school officials will pay close attention to health metrics in the area in coming days as they decide whether to switch students enrolled in in-person instruction back to virtual-only learning.

This group of about 8,000 has been learning through a hybrid model for a few weeks. The students in the cohort had headed back to classrooms gradually throughout October, starting with 40 students on Oct. 5, then adding about 1,000 students on Oct. 19 and approximately 7,000 more on Oct. 26.

The return of these groups does not appear to have caused any sort of super-spreading events. From September through Nov. 8, Fairfax had 128 cases of the virus among staff members and 22 cases among students, according to the most recent data presented to the school board.

Centers for Disease Control and Prevention (CDC) “guidelines allow us to keep small cohorts of students in schools,” Brabrand wrote in his Monday message. “If metrics change that impact in-person instruction [for the students who already have returned], we will update you as soon as that information becomes available.”

He also linked to an online Fairfax information sheet that discusses how coronavirus health numbers relate to reopening decisions.

In the letter to Northam, the teachers’ unions cited summer guidance put out by the CDC that recommended that schools not open if the seven-day positivity rate of new cases per 100,000 people rises above 5 percent. They referenced CDC data showing that the percent of positive polymerase chain reaction (PCR) tests over the past 14 days is above 8 percent Virginia-wide, and above 5 percent in Arlington, Fairfax, Loudoun, Manassas Park and Prince William.

The letter was signed by the education associations for all five counties. It comes as school officials, teachers and parents throughout Northern Virginia continue to battle over the decision to reopen.

In Fairfax County, some teachers’ groups have requested that school remain online-only in the county until the end of the 2020-2021 academic year. At one point, the Fairfax Education Association asked its members to take a day off from teaching for mental health, so they could better work through the decision whether to return to classrooms.

On Monday, the president of another Fairfax teachers’ group, the Fairfax County Federation of Teachers, praised the superintendent’s decision to delay in-person learning, calling it “a good step in the right direction.” But President Tina Williams urged Brabrand to go further, reiterating her group’s request — issued last week — that Fairfax pause all in-person instruction until “there is controlled community spread of COVID-19,” and the school system adopts 11 safety measures outlined by the federation.

In a statement, Williams lamented the fact that the school system has delayed returning the cohort of 8,000 only until the end of November.

“We are shocked that FCPS wants to bring additional groups of students back on November 30th, directly following the Thanksgiving holiday,” she said. “Experts have said this period will be a hot bed for new cases because of expected small group gatherings.”