The Centers for Disease Control and Prevention issued new guidelines for schools Friday that give students who have been vaccinated the option of going maskless this fall while their classmates who have not had shots continue to wear face coverings. 

The much-awaited easing of pandemic rules for kindergarten through 12th grade follows the opening of vaccine eligibility this spring to anyone 12 and older.

But it also arrives after some states and school systems have gone their own way - lifting mask mandates and forging ahead with plans for a highly anticipated school year that is just weeks away.

In Texas, mask mandates were ended in public schools, under the governor's order. In Miami, the familiar face gear is optional this fall. In Marietta, Ga., masks will no longer be required inside buildings or outdoors.

School systems across the country began announcing a few months ago that fall would mark a full return of students to brick-and-mortar schools five days a week - in a major effort to revive at least some pre-pandemic normalcy.

"We recognize that we're about a month out from school starting in some areas," said Erin Sauber-Schatz, a CDC official who oversaw the school guidance. "But we wanted to do a re-review ... to make sure that the recommendations that we were making for the fall school year were based on science and the best available evidence that we have at the time."

Education Secretary Miguel Cardona expressed his support in a statement, saying the top priority in 2021-22 is getting students back into classrooms and school buildings to learn. With federal funding through the American Rescue Plan, he said, "schools have access to unprecedented resources to implement health and safety measures to best accommodate students for full-time in-person learning, and to address our students' social, emotional, and academic needs."

The guidance, which is not binding, reflects many of the same concepts that the CDC released in April and May for the broader population.

But it may fuel a new round of political debate about masking and vaccinations - heralded by some, derided by others - as divisions carry over to practices in schools.

And practically speaking, it may be difficult to implement: It does not spell out how schools would collect accurate information about who is vaccinated - and thus able to go without a mask - and who is not.

"The school has to decide if and how they're able to document vaccination status," Sauber-Schatz said. If that is not possible, she said, "the safest thing to do to protect those people who are not fully vaccinated" is to go with a universal policy requiring masks.

The start of school in August is expected to be far different than a year ago.

Vaccinations are widely available now. Just 13% of the nation's counties are recording high levels of community transmission, compared to 90% in December.

But coronavirus variants, including the highly transmissible delta variant, are a significant unknown for many parents and teachers.

CDC officials say they remain confident in the effectiveness of their "layered" approach to safety in schools - using multiple strategies at once, including masking, distancing, screening, contact tracing, ventilation upgrades, and staying home when sick.

"We really have limited data on transmission of this variant in school settings, but we also don't have any data to suggest that the layered prevention strategies would be ineffective," Sauber-Schatz said.

Still, for some educators, uncertainties about exactly who is vaccinated may translate into a desire for masks for all.

According to CDC data, 24% of people ages 12 to 15 nationally are fully vaccinated, along with 36% of those 16 and 17 years old. Parts of the Washington suburbs have much larger percentages of teenagers at least partly vaccinated.

Vaccinations are not yet available for children under 12, but executives from Pfizer-BioNTech and Moderna have said that data showing whether their vaccines are effective in younger children is expected by fall.

Becky Pringle, president of the National Education Association, called the new guidance "an important roadmap for reducing the risk of covid-19 in schools."

Noting the spread of the delta variant, she said schools should be "consistently and rigorously" using all recommended safety practices, including "requiring masks in all settings where there are unvaccinated individuals present, and ensuring adequate ventilation, handwashing, and cleaning."

While masking was a major change in CDC guidelines, the guidelines stressed that in-person learning is a "priority" for the coming school year, regardless of whether all prevention strategies can be implemented.

It said local jurisdictions should keep a close eye on vaccination rates, community transmission, outbreaks and screening results, as safety measures are weighed or scaled back.

It calls for at least three feet between students in classrooms, but allows for greater flexibility, saying that in schools where not everyone is vaccinated, distancing should happen to the extent possible but "schools should not exclude students from in-person learning to keep a minimum distance requirement." Instead, it says, other prevention strategies, including indoor masking, could be used to help compensate.

The CDC previously revised its school guidelines in March, saying 3 feet of distance between students was sufficient for all elementary and most middle and high schools.

In Maryland, many school leaders have been waiting on the CDC's guidance as they develop plans for fall. In suburban Montgomery County just outside of Washington, masks are mandated indoors for summer school but not required for outdoor activities.

"We're in a holding pattern," said Brenda Wolff, president of the county school board, the day before the guidance was released.

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The Washington Post's Lena H. Sun contributed to this report.