The Centers for Disease Control and Prevention revised its guidelines for schools Friday, saying three feet of distance between students is sufficient for all elementary and most middle and high schools, a change that lays the groundwork for many districts to reopen full-time for in-person classes.

The announcement came as the CDC published new research that found limited coronavirus transmission in schools that require masks but not always six feet of distance, which had been the standard. That was true even in areas with high community spread of the virus.

Teachers unions opposed the change, and local unions may resist efforts to bring large numbers of students back into school buildings at one time. Many big districts have just recently begun to reopen for part-time, in-person school, and often after tense negotiations with teachers.

Nonetheless, the new guidelines represent a significant reversal from CDC guidance issued last month that schools maintain six feet of distance between people. To achieve that, the CDC said, schools in most of the country should hold off on fully reopening.

That put the CDC at odds with President Biden, who has called on schools to fully reopen.

The February recommendation also came under fire from many experts as overly cautious, particularly as more evidence emerged that schools were safely operating with people closer to one another. Nonetheless, with the guidance in place, many districts adopted hybrid systems, where students are in school buildings part of the time and learning from home the rest.

CDC Director Rochelle Walensky said Friday that new evidence prompted the change.

“CDC is committed to leading with science and updating our guidance as new evidence emerges,” she said in a statement. “These updated recommendations provide the evidence-based roadmap to help schools reopen safely, and remain open, for in-person instruction.”

Until recently, the debate over distancing in schools has been complicated by a lack of research directly comparing the risks of various distances between people. Most researchers say the research behind the six-foot parameter is outdated, but they also agree that, in general, more distance is better than less.

An increasing number of scientists have called for smaller distances in schools, saying the risk must be weighed against growing examples of safe reopening and mounting evidence of mental health and academic harms to students who have been learning remotely for more than a year.

“Look, 100 feet is safer than six feet, which is safer than three feet,” former CDC director Tom Frieden said during a Washington Post Live interview this week. “Is three feet okay for most schools? Absolutely, if they mask, if they rapidly identify cases and isolate and quarantine.”

That argument was bolstered last week by a study, published in the journal Clinical Infectious Diseases, that found similar rates of spread in Massachusetts school districts that used a three-feet minimum and in those that used six.

Friday’s change comes after some state and local officials dropped the six-foot recommendation on their own. This week, for instance, Loudoun County Public Schools in Virginia said it would open four days a week in April, a change made possible by reducing the required distance from six to three feet.

Others said the new rules would speed a return to more normal school.

“Like everyone else, we’re waiting for the CDC to change its social distancing guidelines so we can go to three feet,” said Roberto Padilla, superintendent of the Newburgh, N.Y., schools. He said he hopes to ramp up from a hybrid system to full-time this spring.

New York City schools welcomed the news and said on Twitter that it would allow the district to “bring even more students back into buildings!” The district said it would offer a new opt-in opportunity for families next week.

In Philadelphia, Superintendent William Hite also hopes the recommendations will help ramp up in-person teaching, but the district has a long way to go. Currently, an agreement with the teachers union allows only for children in pre-K through second grade in buildings, and only part-time. “We’re going to quickly pivot to reanalyzing distancing for the classrooms and schools so that we can open those rooms and schools to more children,” Hite said Friday.

But the change is opposed by the country’s two large teachers unions, and it’s far from clear that teachers will go along. Ahead of the announcement, the unions argued that there is scant research about the impact of closer contact in urban schools, where buildings are older and classrooms more crowded.

“We are concerned that the CDC has changed one of the basic rules for how to ensure school safety without demonstrating certainty that the change is justified by the science,” said Becky Pringle, president of the National Education Association.

The CDC said Friday that most of the school-based infections have been among adults, or between students and staff. Therefore, it recommended six feet of distance between students and their teachers and among teachers and staff. But the agency said the rules can be relaxed for student-to-student interactions.

For elementary schools, it said, three feet of distance among students is sufficient no matter what the infection rates are in the surrounding community. Young children are much less likely to have severe cases of covid-19, and some research suggests that they spread the coronavirus less efficiently than adolescents and teens.

The recommendations are more complex for middle and high schools and depend on which of four levels of community transmission is present in the surrounding area. At the three lowest levels, the CDC says, three feet of distance is sufficient for all schools. But at the highest tier, the agency recommends six feet — meaning schools would probably have to rotate students in a hybrid system. As of Sunday, CDC data showed that 40 percent of U.S. counties were in that highest tier, defined as 100 or more total new cases per 100,000 people over the previous seven days.

The agency says that even then, secondary schools can drop the standard to three feet, but only if they are able to keep students in cohorts, which limit interactions to a small group. That is difficult to do in middle and high school, where students typically break into different groups depending on the course.

This nuance is helpful, said Joseph Allen, an associate professor at the Harvard T.H. Chan School of Public Health. He said the guidance and the studies published now make clear that in-school infections can be avoided with proper measures in place.

“Even with high community spread, you can control within-building transmission,” he said.

The guidance also made a range of other changes that Allen praised, including a greater emphasis on ventilation and the removal of a recommendation to use Plexiglas and other physical barriers, the use of which he said is not supported by evidence.

The CDC published three new studies Friday that appeared to augment the evidence that schools can operate safely, even where community spread is high, as long as masking and other measures are used.

One, carried out in an urban district of Salt Lake County, Utah, examined transmission in 20 elementary schools over a six-week period in December and January. Mask use among more than 10,000 students and 1,200 staff members was high, and students were grouped in classroom cohorts. The median distance between the children was three feet, the study said.

Fifty-one index, or primary, coronavirus cases were identified, and unlike in previous studies of school-based transmission, the researchers tested all 735 people determined to be close school contacts of the initial 51 — an important step in finding asymptomatic cases that can still transmit the virus. Researchers identified just five cases of in-school transmission, a finding, they wrote, that “strengthens the evidence for low elementary transmission.”

The results, the authors concluded, suggest that even when students are spaced less than six feet apart, “schools in high-incidence communities can still limit in-school transmission by consistently using masks and implementing other important mitigation strategies.”

A second study looked at in-school transmission over two December weeks in 55 K-12 schools in Saint Louis County and Springfield, Mo. Nearly all the schools implemented multiple mitigation measures: masking, ventilation upgrades and hand-washing stations. But spacing between students varied, with many schools using a minimum of three feet. Although community spread was high during the period — with more than 700 daily cases per 100,000 people — researchers identified no school outbreaks and just two cases of in-school spread.

The third CDC study focused on coronavirus cases over four months among school-aged children in Florida, which reopened the majority of schools in August even as most other states kept them closed. It found that youth cases were correlated with rates in the community and that school reopening did not appear to fuel the kinds of spread observed in some group residences or high-density workplaces.

The paper did not address distance between students, but it found that case rates were higher in districts where masks were not required.

A previous version of this article incorrectly described the CDC’s definition of a region with high community transmission. The agency defines it as 100 or more total new cases per 100,000 people over the previous seven days, not a seven-day average of 100 or more daily cases per 100,000 people.