The guidance acknowledges that many students have suffered from months of virtual learning.The Centers for Disease Control and Prevention plans to release new guidance on Friday urging schools to fully reopen in the fall, even if they cannot take all of the steps the agency recommends to curb the spread of the coronavirus — a major turn in a public health crisis in which childhood education has emerged as a political flash point.The agency will also call on school districts to use local health data to guide decisions about when to tighten or relax prevention measures like mask wearing and physical distancing. Officials said they were confident this is the correct approach, even with the spread of the highly contagious Delta variant, and the fact that children under 12 are not yet eligible for vaccination.The guidance, which The Times has seen in draft form, is a sharp departure from the C.D.C.’s past recommendations for schools, bluntly acknowledging that many students have suffered during long months of virtual learning and that a uniform approach is not useful when virus caseloads and vaccination rates vary so greatly from city to city and state to state. The issue of school closures has been an extremely contentious and divisive topic since the outset of the pandemic, and advising school districts has been a fraught exercise for the C.D.C. Virtual learning has been burdensome not only for students but also their parents, many of whom had to stay home to provide child care, and reopening schools is an important step on the economy’s path to recovery.“This a big moment,” said Dr. Richard Besser, a former acting director of the C.D.C. “It’s also a recognition that there are real costs to keeping children at home, to keeping them out of school, that school is so important in terms of children’s socialization and development and it provides other supports as well” — including to working parents.The new guidance will continue to recommend that students be spaced at least three feet apart, but with a new caveat: If maintaining such spacing would prevent schools from fully reopening, they could rely on a combination of other strategies like indoor masking, testing and enhanced ventilation. The guidance recommends masks for all unvaccinated students, teachers or staff members.It also strongly urges schools to promote vaccination, which it called “one of the most critical strategies to help schools safely resume full operations.” Studies suggest that vaccines remain effective against the Delta variant.In previous recommendations, issued in March and reaffirmed in May, the agency said that all schools for students from kindergarten through 12th grade should continue to require masks through the end of the academic year. The agency also said that most students could be spaced three feet apart in classrooms — instead of six feet, which it had recommended earlier in the pandemic — as long as everyone was wearing a mask.“We know that in-person learning is really important for school, for children, for their educational, social and emotional well-being, and so we really want to get kids back in the classroom,” Erin Sauber-Schatz, a captain in the Public Health Service Commissioned Corps who helped lead the C.D.C. task force that wrote the guidance, said in an interview.“Physical distancing is still a recommended strategy,” she added, but she emphasized that if schools do not have sufficient space to keep all students three feet apart, “that should not keep children out of the classroom in the fall.”Socially distanced desks awaited students of the first day of classes in Stamford, Conn., last August.John Moore/Getty ImagesThe guidance relies heavily on the concept of “layered” prevention, or using multiple strategies at once. In addition to masking and social distancing, those strategies may include regular screening testing, improving ventilation, promoting hand washing, and contact tracing combined with isolation or quarantine.The recommendations call on local officials to closely monitor the pandemic in their areas, and suggest that if districts want to remove prevention strategies in schools based on local conditions, they should remove one at a time, monitoring for any increases in Covid-19.Captain Sauber-Schatz said the guidance, which the C.D.C. began drafting in May after the Food and Drug Administration authorized the Pfizer-BioNTech coronavirus vaccines for children ages 12 and older, had “really been written to be flexible.”Reactions among experts were mixed.Jennifer Nuzzo, an epidemiologist at Johns Hopkins University, said that while leaving decisions on school safety protocols to local officials might sound good in theory, it could prove “paralyzing” by putting prevention strategies up for negotiation and debate.“I really hoped they could issue very clear guidelines specifying what level of distance is required,” she said, “and not sort of like a meditative journey on the relative benefits of distance.”Others, including some who have been highly critical of the C.D.C.’s past school guidance, praised the new guidelines.“For the first time, I really think they hit it on the nose,” said Dr. Benjamin Linas, an infectious disease specialist at Boston University. “I think it’s science-based and right on the mark.”Dr. Linas said that he anticipated pushback to the recommendation that unvaccinated children wear masks, but that it still made sense.“I don’t want to send my 11-year-old to school without a mask yet, because Delta is out there,” he said, referring to the highly transmissible variant that now causes the majority of cases in the United States. “And even if she’s not going to get severe Covid from Delta, I’m not ready to take that risk.”Emily Oster, the Brown University economist and author of parenting books who waded last year into the contentious debate over school reopenings, using data to argue that children should return to school in person, said that she was generally pleased with the C.D.C.’s framework, which she said gave districts a road map to reopen without being too prescriptive.Though she had pushed for even more relaxed guidance — doing away with the three-foot rule altogether, for example — she said the new recommendations gave districts important flexibility.“This is, in some ways, the most positive I’ve been about their advice,” Dr. Oster said.Though there are far fewer cases overall than during the winter peak, including in children, they have increasingly made up a greater proportion of cases as the pandemic has gone on and, recently, as more adults have been vaccinated.Children have made up 14 percent of all cases to date, up from around 7 percent this time last year, according to the American Academy of Pediatrics, though serious illness and death among them remains rare.Around 2 percent or less of all coronavirus cases in children result in hospitalization, and even fewer — .03 percent of cases or less — result in death, according to the association’s research. Young children are also less likely to transmit the virus to others than are teens and adults.Still, scientists are concerned about a mysterious inflammatory syndrome that can emerge in children several weeks after they contract the coronavirus, including in those who did not have Covid-19 symptoms at the time of their infection. Some children may also experience lingering, long-term symptoms after being infected, a condition often known as long Covid.Elementary school students in Jersey City, N.J., returning for classes after lockdown in April.Seth Wenig/Associated PressThere are also questions about what role the more contagious Delta variant may play as children and teachers return to the classroom this fall. Captain Sauber-Schatz said that the prevention strategies that have worked for Covid-19 all along also work for the Delta variant, so for now the C.D.C. is “keeping a close eye on it” and will adjust its school guidance if necessary.Last summer, when former President Donald J. Trump was still in office, the White House tried behind the scenes to pressure the C.D.C. into playing down the risk of sending children back to school. The Trump White House also tried to circumvent the C.D.C. and find alternate data showing that the pandemic was weakening, The New York Times reported in September.In May, the agency created some confusion, including among parents and educators, in when it abruptly changed its guidance on mask-wearing and announced that vaccinated people could go without masks in most indoor and outdoor settings.The agency then clarified its advice for schools and recommended universal use of masks and physical distancing in the classroom through the end of the school year.The new guidelines still rely on quarantine as a prevention strategy for unvaccinated students when they are exposed to the virus, which Dr. Oster criticized as a significant hindrance for students and parents, even as research has consistently suggested that transmission in schools is low.“It’s really disruptive,” Dr. Oster said of quarantine requirements.In the previous guidelines, physical separation was contentious, and the new version may not resolve the debate. While the C.D.C. recommends that students be permitted to sit just three feet apart, it continues to call for teachers and other staff members to remain at least six feet away from students regardless of their vaccination status — and if they are unvaccinated, six feet away from one another.Captain Sauber-Schatz said those recommendations were rooted in science.“For the studies that have been done looking at the difference between three feet and six feet, those were all between students in the classroom, not between teachers and students,” she said. “We have the science and the evidence to make that recommendation, that three feet is permissible between students in the classroom. We don’t have that level of evidence for the staff.”