Ideally, the C.D.C. should also have mentioned high-quality masks or double-masking, Dr. Allen said. (The agency on Thursday released new advice for masking that included the use of two masks at once.)

Other preventive measures the C.D.C. recommended for schools are those it has previously endorsed: universal masking of staff and students; physical distancing; hand-washing and hygiene; cleaning; and contact tracing, in combination with isolation for those who have tested positive and quarantine for those who have been exposed to the virus.

The agency advised that schools refer all symptomatic students, teachers, staff and close contacts for diagnostic testing, and that schools put in place routine weekly testing of students and staff, except when community transmission is low. But the expense and logistics of widespread screening would be a heavy burden for school districts.

The C.D.C. skated lightly over physical distancing. The agency’s previous recommendation for distancing suggested that schools have students attend on alternating schedules, in order to reduce the number of students in classrooms and hallways.

The new guidance instead says schools should put in effect physical distancing “to the greatest extent possible,” but requires it only when community transmission of the virus is high. The softer emphasis makes the guidelines more feasible for school districts to follow, Dr. Nuzzo said.

“A lot of communities have pursued hybrid approaches, or in some cases just not opened, because they haven’t been able to figure out that spacing issue,” she said. The guidelines give the impression that maintaining at least six feet of distance between students is ideal, “but the whole attempt to bring kids back to school doesn’t have to break down over that,” she added.

The six-feet rule has been embraced as an orthodoxy, however, by many educators. Becky Pringle, president of the National Education Association, the country’s largest teachers union, said there should be no wiggle room on physical distancing or other mitigation strategies.

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