Students in Montgomery County will not return to in-person classes until March 15 at the earliest, under a new schedule that marks another delay in classroom-based instruction amid the surging pandemic.

The school board approved the change Tuesday in a 7-to-1 vote, with officials in the state’s largest school system warning that health benchmarks remain key. They also said state or federal guidance about schools could shift as coronavirus vaccines are more widely distributed.

The decision means that most of Montgomery’s more than 161,000 students will have been away from the once-familiar setting of their classrooms and schools for a full calendar year.

It also means Montgomery County students will begin the second semester of an unprecedented school year in the same place they started the first semester — at home, on laptop computers.

“I know this is difficult for some, and for others they’re relieved,” said Jack R. Smith, superintendent of the suburban Maryland school system. “I understand those emotions. . . . We think this makes the most sense at this point.”

Parents in Montgomery have been increasingly vocal about reopening schools as instruction has remained all-virtual month after month.

Many have argued that school officials have dragged their feet in getting students back on campus. They point to the experiences of private schools, child-care centers, equity hubs and public school systems that have gotten at least some students inside classrooms.

Other parents and teachers have pointed to the worsening pandemic. Health and safety must be the top priority, they assert.

Emails to school officials have poured in, citing research, mental health fallout, learning loss, the needs of students in special education and a range of potential consequences for teachers and families. Some messages have been angry. Others have been fearful or worried.

“I know there is going to be anger and frustration,” said board member Patricia O’Neill (District 3). “We are talking about one year. But it’s a very, very difficult situation.”

On Tuesday, the two-week average number of new cases per 100,000 residents in Montgomery County was 44, with a test positivity rate of 7.5 percent. Montgomery’s targets for reopening are under 15 cases per 100,000 and below 5 percent.

“Both of those are significantly above the thresholds that the board set in December,” said Derek Turner, chief of engagement, innovation and operations.

But some are hoping that vaccine distribution will bring significant change. In late January, the first teachers and other front-line school employees are scheduled to get their first doses of the vaccine.

“I do wholeheartedly believe that vaccination is the path forward to a normal existence,” O’Neill said.

Smith said the school board would meet Feb. 23 to determine whether the March opening is feasible. The second semester begins Feb. 1.

Montgomery’s discussion Tuesday came as teachers unions across the Washington region called for school systems to come up with a regionwide plan to return students to school buildings.

With reopening efforts so varied, it has been hard on parents and school employees who struggle with child-care needs and financial uncertainty, they said.

Coming together to make the case were more than 10 unions or associations — in Virginia, Maryland and D.C. — representing more than 48,000 public school educators, they said. They included the teachers union in Montgomery County.

“We live in one of the most populous regions of America,” the groups said in a statement. “The lack of a coordinated plan among the many school districts in the region is creating instability and unnecessary anxiety for parents, children, and school employees in the Washington DC metro area.”

Since the school year began in August, school systems in the region have made and changed plans multiple times.

For now, most students in Northern Virginia are scheduled to begin the second semester entirely online. Neither Loudoun County’s school system, with 82,000 students, nor Fairfax County’s, with 186,000 students, has committed to a firm return date.

In Alexandria, the school system of 16,000 on Monday delayed by a week the return of a group of kindergartners through second-graders with disabilities. Because of climbing infection rates, the children are not expected to return until Jan. 26.

In Arlington, 230 children with disabilities have been learning in-person since November, but it is unclear when other students will be back. The school system, which enrolls 23,000, has asked some of its teachers and administrators to return over the next month.

Prince George’s County in Maryland, which has been all-online since the school year began, is sticking with virtual learning until further notice, a spokeswoman said. Officials hope that some form of in-person learning may resume before the school year’s end in the state’s second-largest school system.

At Tuesday’s meeting in Montgomery, officials outlined a set of changes aimed at helping struggling students in middle and high school.

Students will be allowed to designate up to two courses for pass-fail grades in the second semester rather than letter grades. They also can lighten their course load — opting to take a class during the summer or another time in the future.

School system officials said they will do more to provide tutoring and outreach, and that the system will offer more support during Wednesday virtual check-ins. The changes follow several adjustments made for the second marking period.

School system data shows failure rates in math and English jumped as much as sixfold during the first quarter for some of Montgomery County’s most vulnerable students.

More than 36 percent of ninth-graders from low-income families, for example, failed the first marking period in English. That compares with fewer than 6 percent last year, when the same students took English in eighth grade.

Hannah Natanson contributed to this report.