Classroom-based learning in schools across the Washington region could expand for spring and look more normal by fall, following changes in safety guidance announced Friday by the Centers for Disease Control and Prevention.

Federal officials announced revisions that allow for three feet of separation between students in most cases — a change that could remove one of the most significant obstacles to bringing more students into schools at this juncture of the pandemic.

It came amid new CDC research that showed limited transmission of the virus in schools where students wear masks but do not always maintain six feet of distance. The finding held up even during high levels of spread of the virus.

Some parents jumped on the changes, seeing them as a major spark for speeding the move to in-person learning. But school systems took a more measured approach, even as the issue has been a central focus of reopening plans.

In Maryland’s Montgomery County, Superintendent Jack R. Smith said last week that the sprawling school system is planning for a five-day in-person week in the fall, if social distancing requirements are relaxed so that more students can return at once.

The new guidance appears to pave the way.

Montgomery County school officials issued a statement welcoming the new guidance and saying they would continue to work closely with health officials and members of the school community to safely implement the new measures.

No changes are anticipated for spring “at this time,” but the guidance could affect the fall, said spokeswoman Gboyinde Onijala.

Parents in Montgomery County who have been pressing for reopening school buildings in the state’s largest school system expressed dismay, saying a sense of urgency is needed.

“Every day that students are subjected to online learning is another day that is adding to the mental health crisis, the academic and physical health crisis,” said parent Margery Smelkinson, an infectious-disease scientist who is a leader in the parents organization Together Again MCPS. “Obesity rates are rising. Grades are suffering and kids are dropping out. This is a national crisis.”

School systems in the region have been moving to bring back students at varied paces — with at least one school district ahead of the federal guidance.

Just this week, Virginia’s Loudoun County surprised many educators and parents by opting for three feet of in-person separation — from “nose-to-nose” — as part of a plan to send students into classrooms for four full days a week in April. The system had been relying on a six-feet standard.

Scott A. Ziegler, Loudoun’s interim superintendent, said in an email to families that he hoped “this step will bring us all a little closer to the normalcy that we crave.”

The plan, to start April 20, stands to affect nearly 30,000 Loudoun students who returned to in-person learning, for a couple of days a week, during the past month. Most of the system’s 81,000 students are still learning from home.

Asked Friday how the CDC’s new rules affect Loudoun’s plans, spokesman Wayde Byard gave a two-word answer: “No changes.”

In nearby Alexandria City Public Schools, the CDC guidance likely means the district will be able to return more students to in-person learning. The school system of 16,000 sent roughly 5,300 children back into classrooms for two days of instruction a week over the course of the past month.

At a school board meeting Thursday night, Alexandria Superintendent Gregory C. Hutchings Jr. said that if the CDC revises its recommendations to three feet of distance, his school system will send additional children into classrooms after spring break. He said that there is significant parent demand to switch their remote learners to in-person schooling, and that Alexandria will have to prioritize returning students who are struggling academically, socially and emotionally, as well as children with disabilities and those whose first language is not English.

Alexandria officials did not respond to a question asking how the CDC revision affects their plans.

School officials in Arlington and Fairfax County, meanwhile, said their school systems are going to stay the course for now, as staffers plot out the implementation of the three-foot rule. Both school divisions spent the past month returning roughly half of their student bodies to classrooms under a program that keeps them six feet apart from peers. Both have vowed to return to five days a week of in-person instruction this fall.

“We are closely reviewing the updated guidelines to determine how it may impact our ability to increase classroom capacities and improve transportation in the current school year,” said Arlington spokesman Frank Bellavia.

Fairfax Superintendent Scott Brabrand said Friday in a message to parents that the new guidance was “very encouraging,” but under review. He said he will meet with principals and county health officials next week to work on the issue.

“We will be proposing next steps once we have completed a thorough review,” Brabrand said.

But some pro-reopening parent groups in both Arlington and Fairfax are calling for action now. A group known as Arlington Parents for Education put out a statement asking Superintendent Francisco Durán to reduce the distance between students to three feet, increase the number of in-person days each week and expand bus capacity.

And the group calling itself Open Fairfax County Schools called on Brabrand to start sending children to school for five days a week immediately.

“There are no more excuses,” the group wrote in an open letter. “It is time to make it work.”

Prince George’s County school officials said no changes are planned as a result of the new distancing guidance. The school system, the second-largest in Maryland, plans to phase in 42,000 students who want to return to campus for hybrid learning in April.

In D.C., school officials said once they receive the health department’s updated guidance, they will evaluate how changes should be made “while prioritizing community engagement and the safety of our students.”

“It remains our firm belief that the best place for students to learn is in the classroom,” the school system said in a statement Friday.

Earlier this week, D.C. Health Director LaQuandra Nesbitt loosened some guidance, and removed the cap size of students in each classroom, so long as social distancing rules are followed. She also said cohorts could mix and students could switch classes during the day, though she warned of greater risks of spreading the virus.

The school system’s agreement with its teachers union — which took months to negotiate — says schools should align with health guidelines but also explicitly calls for six feet between student desks. It’s unclear whether teachers will be willing to relax that standard.

Many parents in the District are pushing for the city to reopen schools for everyone and change the social distancing rules to three feet, arguing that the science is on their side. A large group organized a rally in D.C. last weekend calling for the changes.

The six-foot rule “is being used as an excuse to hide behind and keep our schools closed,” an organizer said through a megaphone at the rally.

But there was no representation from the wards with the highest concentrations of poverty. Parents in these neighborhoods may be more reluctant to return with a change in guidance. Principals have said that one of the first questions parents ask them when deciding whether to sign up for an in-person slot is how many students will be in the class.

In Montgomery, teachers union leaders said they were still reviewing the revisions. Asked about its agreement with teachers, Montgomery officials said there is no provision for six feet of distancing in the system’s memorandum of understanding.