The Atlanta-based Centers for Disease Control and Prevention issued new guidelines on Friday that recommend students in classrooms can now distance at three feet, rather than the formerly recommended six.

Masks are remaining mandatory, the CDC said, and teachers and other adult school staff must still adhere to the six feet guidelines.

Earlier this week, research published in the Clinical Infectious Diseases looked at schools in Massachusetts, which has kept students three feet apart, instead of the recommended six, for several months. Illinois and Indiana also are allowing three feet of distance.

President Joe Biden’s top health advisor, Dr. Anthony Fauci, told NBC’s Today he expected the CDC to reduce the recommended guidelines from six to three feet.

On Wednesday, CDC Director Dr. Rochelle Walensky confirmed the agency is reviewing the recommendations, acknowledging the six-foot guideline is “among the biggest challenges” schools have faced in reopening.

Even as more teachers receive vaccinations against COVID-19, social distancing guidelines have remained a major hurdle for districts across the U.S. Debate around the issue flared last week when a study suggested that masked students can be seated as close as three feet apart with no increased risk to them or teachers.

The CDC included the larger spacing in its latest school guidelines, which were issued in February and concluded that schools can safely operate during the pandemic with masks, distancing and other precautions. It suggested six feet and said physical distancing “should be maximized to the greatest extent possible.”

Other organizations have issued more relaxed guidelines, including the World Health Organization, which urges one meter in schools. The American Academy of Pediatrics says to space desks “three feet apart and ideally six feet apart.”

Dan Domenech, executive director of AASA, a national superintendents group, said he expects more states and schools to move to the three-foot rule in coming weeks. With the larger guideline, he said, most schools only have space to bring back half of their students at a time. Moving to three feet could allow about 75 percent at a time, he said.

“There are districts that have been doing three feet for quite some time without experiencing any greater amount of infection,” he said.

In Illinois, health officials said last week that students can be seated three feet apart as long as their teachers are vaccinated. Before, state officials required six feet.

With the state’s blessing, the Barrington district near Chicago reopened middle schools Tuesday using the smaller spacing rule. Any student will be allowed to attend in-person classes, although the district expects roughly 30 percent to continue with remote learning.

Questions around spacing have led to a battle in Massachusetts, where teachers and some schools oppose a state plan to bring younger students back five days a week starting next month. The plan calls on schools to seat students three feet apart, although many have been using six feet as a standard. Districts that fail to meet the reopening deadline would risk losing state funding.

The Massachusetts Teachers Association, a statewide union, argues that seating students closer will increase the risk for everyone in the classroom. It also poses a problem for districts that have agreed to contracts with teachers adopting the six-foot rule as a requirement.

“They can’t just throw six feet out the window. They can’t throw away what has been agreed upon,” said Merrie Najimy, president of the union. “If they can’t make it work, then they’re going to have to come to a new agreement.”

Public schools in Worcester are among those pushing back against the closer spacing. Tracy O’Connell Novick, a member of the district school committee, said switching to the three-feet standard would be “betting the health” of thousands of students and staff.

In Boston’s public schools, desks will be spaced at least three feet apart, but teachers and staff will be asked to keep six feet from students and other staff when feasible, district spokesperson Xavier Andrews said. Schools will also use larger rooms and outdoor spaces to keep students at a safe distance, he said.

In some states that already allow three-feet spacing, schools say they have seen no evidence of increased risk. School officials in Danville, Indiana, which moved to three feet in October, said students have been in the classroom all year with no uptick in virus transmission.

“It’s gone very well for us. I won’t say there has been no transmission, but it’s been staggeringly low — like one time or something like that,” said Tim McRoberts, vice president of the school board. “We’ve kept our doors open. We’ve had no temporary shutdowns.”

In Ohio, Cincinnati’s school board got an earful from parents and others last month when it proposed resuming in-person learning at the crowded Walnut Hills High School under a model that called for distancing of only three feet there while its other schools would use 6 feet.

The critics included Walnut Hills teacher Brandon Keller, who said the plan was dangerous. He warned the board that their decision “will have a body count.”

Board members backed off on reopening that school, then weeks later narrowly voted for a plan that included a phased reopening, but they also warned that the physical distancing might be less than 6 feet. Students also have options to continue learning virtually.

Seven superintendents in central Oregon sent a letter to Gov. Kate Brown last week asking the state to relax some of its social distancing rules — including the six-foot barrier — so that more students can return to class full time.

Oregon’s Crook County School District, which has had students in classrooms most of the school year, has found that masks, contact tracing and sending students home when they show symptoms are the most effective means of combating the virus.

“The six-feet rule doesn’t make as much sense as the other safety measures,” district spokesperson Jason Carr said. “What may have made sense two months ago or at the beginning of the year might not now.”

The Associated Press contributed to this report.