BOSTON (AP) — New evidence that it may be safe for schools to seat students 3 feet apart — half of the previously recommended distance — could offer a way to return more of the nation’s children to classrooms with limited space.

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 3 feet apart with no increased risk to them or teachers.

Published in the journal Clinical Infectious Diseases, the research looked at schools in Massachusetts, which has backed the 3-foot guideline for months. Illinois and Indiana are also allowing 3 feet of distance, and other states such as Oregon are considering doing the same.

The Centers for Disease Control and Prevention is now exploring the idea too. The agency’s director, Dr. Rochelle Walensky, said the 6-foot guideline is “among the biggest challenges” schools have faced in reopening.

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 6 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 1 meter in schools. The American Academy of Pediatrics says to space desks “3 feet apart and ideally 6 feet apart.”

Dan Domenech, executive director of AASA, a national superintendents group, said he expects more states and schools to move to the 3-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 3 feet could allow about 75% at a time, he said.

“There are districts that have been doing 3 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 3 feet apart as long as their teachers are vaccinated. Before, state officials required 6 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% 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 3 feet apart, although many have been using 6 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 6-foot rule as a requirement.

“They can’t just throw 6 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 3-feet standard would be “betting the health” of thousands of students and staff.

In Boston’s public schools, desks will be spaced at least 3 feet apart, but teachers and staff will be asked to keep 6 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 3-feet spacing, schools say they have seen no evidence of increased risk. School officials in Danville, Indiana, which moved to 3 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 3 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 6-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 6-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.”

Associated Press writers Philip Marcelo, John Seewer and Kantele Franko contributed to this report.