A Northern California school district that is allowing students to go unmasked was forced to cancel classes Thursday because of teacher absences.

The district that oversees Nevada Union High School in Grass Valley announced Monday that masks would, in effect, be optional. It said enforcement would consist of "educating students and asking them to mask" and would no longer involve exclusion from classes or other school activities.

On Wednesday, dozens of teachers were absent, causing the cancellation of many classes. Students were sent to an auditorium to watch a movie instead, The Union reported. On Thursday, school was closed because of teacher absences, the principal announced.

District superintendent Brett McFadden said the mask decision was prompted by "the district's inability to effectively enforce the state's mask mandate" in light of what he called "significant civil disobedience" by students and parents.

The previous week, about 50 protesters — including students skipping class — had gathered at the district office to express their unhappiness with the continued mask mandate, which was in line with statewide guidelines.

After the district changed its policy, the teachers' union issued a statement decrying "violation of the contract and the collective bargaining process."

The school, which serves the Sierra foothills area including Grass Valley and Nevada City, has about 1,500 students.

Nevada County has been a hotbed of protests against COVID-19 restrictions, with some opponents launching a bid to recall all five county supervisors over the issue.

Gov. Gavin Newsom is expected to make a statement Monday on whether masks will continue to be required in the state's schools. The mask mandate was lifted last week for most fully vaccinated Californians.