In another postponement, the CDC made it clear that masks for vaccinated individuals could be optional for the general public as per its recommendations.

Still, the agency said schools could opt for universal masking if, for example, local cases increased or if a school couldn’t determine how many of its students and staff were vaccinated. And it urged schools to “support people who are fully vaccinated but choose to continue wearing a mask”. In general, students and employees do not need to be masked outdoors, the agency said.

The CDC also urged schools to promote vaccination, which the guidelines identify as “one of the most important strategies for safely resuming full operation in schools”. Studies suggest that vaccines against the Delta variant remain effective.

The country’s two major teachers’ unions, which have close ties with the Biden government, praised the guidelines. Randi Weingarten, president of the American Federation of Teachers, whose members campaigned against reopening schools in some cases last school year, said the recommendations were “based on both science and common sense.”

Still, both school and health authorities predicted the challenges ahead.

Ms. Weingarten said the mask instructions were a special test as classes with students aged 12 and over would most likely include a mix of vaccinated and unvaccinated students. Many officials in areas with low vaccination rates have already stated that they do not need masks in schools – and at least eight states have already banned such requirements.

Updated

July 10, 2021, 12:53 p.m. ET

Some parents who voted to reopen the school welcomed the new guidelines with relief. Meredith Dodson, whose son is starting kindergarten in San Francisco this fall, organized a group of parents who spent the final year fighting for the city to open their schools. The city finally allowed elementary school students to return in mid-April, but most middle and high school students couldn’t do this at all.

“This is a big step in the right direction,” said Ms. Dodson.

Many schools have already largely or completely returned to face-to-face teaching. By mid-spring, the vast majority of districts had allowed at least younger students to return to class, although many, particularly on the west coast, allowed them only part-time classes. Many families – especially Asian-American, Black, and Hispanic families – have chosen to let their children study remotely.