The Centers for Disease Control and Prevention has called for the full reopening of K-12 schools in the fall — even if not all preferred COVID-19 safety measures can be upheld.

“Students benefit from in-person learning, and safely returning to in-person instruction in the fall 2021 is a priority,” the agency wrote in new guidance issued Friday.

The CDC still recommends schools maintain “at least 3 feet of physical distance” between students indoors, but the agency included a surprising caveat.

“When it is not possible to maintain a physical distance of at least 3 feet, such as when schools cannot fully re-open while maintaining these distances, it is especially important to layer multiple other prevention strategies, such as indoor masking,” it wrote.

However, the agency conceded that fully vaccinated kids do not need to wear masks inside classrooms.

Other “important layers of prevention” include testing, ventilation, handwashing and contract tracing, the agency said.

“Many schools serve children under the age of 12 who are not eligible for vaccination at this time. Therefore, this guidance emphasizes implementing layered prevention strategies … to protect people who are not fully vaccinated, including students, teachers, staff, and other members of their households,” the health agency wrote.

The heavy emphasis on “layered prevention” in the guidance is the strongest acknowledgment yet from the CDC that schools need to prioritize a return to in-person learning.

Still, New York City’s teachers’ union expressed caution about the new guidance Friday.

“We will be reviewing the CDC’s recommendations and having conversations with our independent medical experts, as well as meeting with the city and state agencies that are responsible for deciding how to proceed,” said United Federation of Teachers boss Michael Mulgrew.

However, Schools Chancellor Meisha Ross-Porter said the new guidelines mesh with current DOE policy.

“New York City schools led the nation by safely opening our doors when no other major district did, and we will welcome our students back to full-time, in-person learning in September,” she said.

Ross-Porter said the DOE will now review the CDC advisory with city health officials in formulating a return to school in the fall.

In May, the CDC approved Pfizer’s COVID-19 vaccine for kids ages 12 to 15 — opening up the livesaving shots to millions of young Americans.

No vaccines have yet been approved for kids under the age of 12.