New York City schools have been temporarily blocked from enforcing a vaccine mandate for its teachers and other workers by a federal appeals judge just days before it was to take effect.

Workers in the nation’s largest school system were to be required to show vaccination proof starting Monday. But late Friday, a judge for the 2nd U.S. Circuit Court of Appeals granted a temporary injunction sought by a group of teachers and referred the case to a three-judge panel an an expedited basis.

Department of Education spokesperson Danielle Filson said that officials are seeking a speedy resolution and that the circuit court has the motion on its calendar for Wednesday.

“We’re confident our vaccine mandate will continue to be upheld once all the facts have been presented, because that is the level of protection our students and staff deserve,” Filson said in an email.

The New York Post reported that the department sent an email to principals Saturday morning saying they “should continue to prepare for the possibility that the vaccine mandate will go into effect later in the week.”

Mayor Bill de Blasio announced in August that about 148,000 school employees would have to get at least a first dose of the COVID-19 vaccination by Sept. 27. The policy covers teachers, along with other staffers, such as custodians and cafeteria workers.

It’s the first no-test-option vaccination mandate for a broad group of city workers in the nation’s most populous city. And it mirrors a similar statewide mandate for hospital and nursing home workers set to go into effect Monday.

As of Friday, 82% of department employees have been vaccinated, including 88% of teachers.

Even though most school workers have been vaccinated, unions representing New York City principals and teachers warned that could still leave the 1 million-student school system short of as many as 10,000 teachers, along with other staffers.

De Blasio has resisted calls to delay the mandate, insisting the city was ready.

“We’ve been planning all along. We have a lot of substitutes ready,” the Democrat said in a radio interview on Friday. “A lot is going to happen between now and Monday but beyond that, we are ready, even to the tune of, if we need thousands, we have thousands.”