The Centers for Disease Control and Prevention on Friday urged that K-12 schools be reopened and offered a comprehensive science-based plan for doing so speedily, an effort to resolve an urgent debate roiling in communities across the nation.

The new guidelines highlight the growing body of evidence that schools can openly safely if they put in effect layered mitigation measures. The agency said that even when students lived in communities with high transmission rates, elementary students could receive at least some in-person instruction safely — a finding echoed by an independent survey of 175 pediatric disease experts conducted by The Times.

Middle and high school students, the agency said, could attend school safely at most lower levels of community transmission — or even at higher levels, if schools put into effect weekly testing of staff and students to identify asymptomatic infections.

Among the pediatric experts surveyed by The Times, the point of most agreement was requiring masks for everyone: students, teachers, administrators, and other staff. All respondents said universal masking was important, and many said it was a simple solution that made the need for other preconditions to opening less essential.

“C.D.C.’s operational strategy is grounded in science and the best available evidence,” Dr. Rochelle Walensky, director of the C.D.C., said on Friday in a call with reporters.

The guidelines arrive in the middle of a debate that is already highly fraught. Some parents whose schools remain closed are becoming increasingly frustrated, and public school enrollment has declined in many districts across the country.

Education and civil rights leaders are despairing about the harm being done to children who have not been in classrooms for nearly a year. And many of the pediatric health experts also expressed deep concern about other risks to students of staying home, including depression, hunger, anxiety, isolation, and learning loss.

“Children’s learning and emotional and, in some cases, physical health is being severely impacted by being out of school,” said Dr. Lisa Abuogi, a pediatric emergency medicine physician at the University of Colorado, expressing her personal view. “I spend part of my clinical time in the E.R., and the amount of mental distress we are seeing in children related to schools is off the charts.”

The Biden administration has made a high priority of returning children to classrooms, and the new recommendations try to carve a middle ground between school officials as well as some parents who are eager to see a resumption of in-person learning and powerful teachers’ unions resisting a return to school settings that they regard as unsafe amid the coronavirus pandemic.

Whether the guidelines will persuade powerful teachers’ unions — allies of Mr. Biden — to support teachers returning to classrooms remains to be seen. In advice that may be disappointing to some unions, the document states that, while teachers should be vaccinated as quickly as possible, teachers do not need to be vaccinated before schools can reopen.

“I completely understand teachers’ and other school employees’ fear about returning to school, but there are now many well-conducted scientific studies showing that it is safe for schools to reopen with appropriate precautions, even without vaccination,” said Dr. Rebecca Same, an assistant professor in pediatric infectious disease at Washington University in St. Louis. “They are much more likely to get infected from the outside community and from family members than from school contacts.”

The C.D.C. document embraces the often-repeated mantra that schools should be the last settings to close in a community and the first to reopen. But that has been followed nowhere in the country, and these guidelines have no power to force communities where transmission remains high to take steps, such as closing nonessential businesses, to decrease it.

As a result, some teachers’ unions will continue to argue that the overall environment remains unsafe to return to in-person classrooms.

A majority of districts in the country are offering at least some in-person learning, and about half of the nation’s students are learning in classrooms. But there are stark disparities in who has access to in-person instruction, with urban districts, which serve mostly poor, nonwhite children, more likely to be closed than non-urban ones.