A recent spike in covid cases among children in the U.S. has spurred the country’s second largest teachers union to endorse vaccination mandates for educators as schools prepare for face-to-face learning this fall, said Randi Weingarten, president of the American Federation of Teachers , on Wednesday.

“That really scares me: In the last three weeks, the number of children who tested positive has increased from 20,000 to 40,000 to 72,000,” she said, citing data from July. The number of children who tested positive for Covid in the week ending August 5 was even higher at 93,824, according to the latest data from the American Academy of Pediatrics.

Weingarten, who spoke to CNBC’s “Squawk Box” in an interview, said schools should give teachers time to get the shots and allow medical and religious exemptions for those who don’t want them.

“Children under the age of 12 cannot get vaccines, this delta virus is very transmissible, so we have to be in school for our children with our children, but we have to protect everyone,” said Weingarten. “And that means vaccines are the most important way, and the second way to do it is with masks.”

About 90% of teachers have already been vaccinated, Weingarten said during the interview, citing data from the White House. However, since many children can still not be vaccinated, Weingarten has not been able to advocate compulsory vaccination for students under 12 years of age.

With the rise of the Delta variant, states have begun improving their Covid containment protocols to prevent the virus from spreading among faculties and students. On August 4, Illinois Governor JB Pritzker made mask compulsory for all state students regardless of their vaccination status.

New Jersey also enacted a mask mandate for all students and staff on Friday, and Louisiana’s mask mandate for indoor public spaces includes students from kindergarten through college.

Becky Pringle, president of the largest US teachers’ union, the National Education Association, told the New York Times last week that any vaccination mandate should be negotiated locally.

– Reuters contributed to this report.