The leader of the nation’s second-largest teachers’ union is “not convinced that the evidence supports” easing social distancing requirements in schools, a shift many policymakers have seen as key to getting more students into classrooms for in-person learning.

The Centers for Disease Control and Prevention said last week that 3 feet of space between studentswho are wearing masks is a sufficient safeguard in most classroom situations, a change from a long-running recommendation of 6 feet of social distancing in schools.

“Weakening one layer of layered mitigation demands that the other layers must be strengthened,” American Federation of Teachers President Randi Weingarten said in a Tuesday letter to U.S. Secretary of Education Miguel Cardona and CDC Director Rochelle Walensky.

“We strongly urge you, in any discussion of this shift, to forcefully insist on strict and strengthened adherence to the other mitigation strategies,” like mask-wearing, testing, and ventilation, she wrote, asking for more guidance and research on the issue.

Concerns from teachers unions, considered a key ally of President Joe Biden, come as Cardona, Walenksy, and other officials prepare for a Wednesday summit on school reopening, a key part of the administration’s efforts to encourage more in-person learning.

The 6-feet social distancing rule has been one of the most consistent prevention strategies during the pandemic. But policymakers and educators said it made it very difficult to open schools to all students. And CDC officials pointed to new research that suggested students may be just as safe with less space if they took other precautions, like consistently wearing masks.

The CDC modified its recommendations Friday, a little more than a month after the agency released broader updated for schools, seeking to regain credibility and consistency in its messaging to schools under the newly formed Biden administration.

The new guidelines say 6 feet of space is still necessary in middle schools and high schools in communities with high transmission rates unless schools can group students in small cohorts that remain together throughout the school day.

Adults should also maintain 6 feet of space from each other and from students, the recommendations say. And 6 feet of space is still necessary between students in common areas, like lobbies; in situations where masks can’t be worn, such as meal times; and when “increased exhalation occurs,” like during sports, choir and band rehearsals, and exercise.

The decisions last week were met with terse statements from the two national teachers’ unions, which both promised to review the research the CDC cited in support of its changes.

“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 and can be implemented in a manner that does not detract from the larger long-term needs of students,” National Education Association President Becky Pringle said in a statement Friday.

AFT’s Weingarten said in her letter Tuesday that she remained concerned, even after she took the time to further review the research.

“Our concern is that the cited studies do not identify the baseline mitigation strategies needed to support 3 feet of physical distancing,” she wrote to Cardona and Walensky. “Moreover, they were not conducted in our nation’s highest-density and least-resourced schools, which have poor ventilation, crowding and other structural challenges.

Any reduction in distancing must be accompanied by “at a minimum, universal and correct masking; effective ventilation; thorough cleaning of buildings; regular COVID-19 testing of teachers, staff and students; effective contact tracing and quarantine/isolation protocols; and the availability of vaccines to all people in schools who are eligible,” Weingarten wrote.

She also raised a list of logistical concerns, including how a teacher would maintain six feet of space, even if students are closer together; how distancing rules will apply to small group settings, like reading circles, and on school buses; and the timing of the change.

“Many school systems are just returning to in-person instruction right now, after significant planning—for bus routes, staggered schedules, etc.—based on 6 feet of physical distancing,” she wrote. “Even with the significant investment of American Rescue Plan money, districts lack the human resources and institutional planning ability to make changes like this quickly. Is this something that can be implemented in the fall, or perhaps the summer?”

Many of those decisions are in the hands of state and district leaders, not federal agencies. The CDC’s guidance is nonbinding and designed to inform state and local directives, many of which have already differed in some key areas.

At a media briefing last week, Walensky said the rest of the CDC guidelines for schools remain in place, and she especially stressed the importance of “universal and proper mask wearing.”

Walensky cited studies released by the CDC Friday that showed relatively low rates of COVID-19 transmission in schools, even when there were high infection rates in their broader communities. But most of the studies cited did not specifically analyze the difference in transmission rates between students spaced between 3 feet and 6 feet apart; rather, they noted that as a practical matter, most schools in the studies had been unable to meet the existing 6-foot spacing guidelines.

Since the CDC announced the change, state and district leaders around the country have said they plan to reexamine their recommendations to schools. In New York City, Mayor Bill de Blasio said the city will give students who are learning remotely another chance to opt in to in-person instruction in light of the new guidelines.

But the previous social distancing recommendation was already incorporated into reopening plans and agreements with teachers’ unions around the country. It’s unclear how much the change will affect those plans.

Trackers maintained by several private organizations show most students already have the option of either fully in-person instruction or a hybrid of in-person and remote learning. Data maintained by Burbio, a company that tracks community events, shows an estimated 18.1 percent of U.S. K-12 students attended fully virtual school last week.