Striking teachers in Arizona are facing criticism as more than 1,000 of the state’s public schools remain closed Monday.

The Phoenix-based Goldwater Institute has informed teachers their strike is illegal and deprives Arizona students of their constitutional right to a guaranteed education:

As Education Week observes, Goldwater Institute is charging that closed school districts are part of a “coordinated plan” to allow teachers to leave their jobs without penalty.

“This letter is to inform you and all district employees that these acts are unlawful,” a letter from the Goldwater Institute states. “If the district does not reopen and employees do not return to their duties, parents and students will have a legal cause of action against them.”

The letter urges school districts to reopen public schools with substitute teachers to avoid a lawsuit.

In a blog post on the Goldwater Institute’s website, Vice President for Litigation Timothy Sandefur noted school districts are so complicit in the strike that one district even changed its employment policy Sunday night to allow teachers to take a sick day without a doctor’s note.

Teachers in the Grand Canyon State banding together under the #RedForED movement for the third day are using the Twisted Sisters’ tune “We’re Not Gonna Take It” as their fight song, reports AZcentral.com. Teachers are demanding higher pay and more school funding.

Rebecca Garelli, an organizer with Arizona Educators United, said the strike is “a day by day operation” and that educators should expect little notice about whether her group will continue the walkouts for another day.

The union’s demands are “concrete,” said organizer Noah Karvelis, 23, who has advocated for teaching his music students political activism via Marxist philosophy.

“The demands have not changed since day one,” he said, according to AZcentral.com. “Those come from a place of educators saying …  this is what the change needs to look like. Those are concrete. Those are not moving.”

Teacher demands include a 20 percent salary increase and a restoration of education funding to 2008 levels, a change that would require adding approximately $1 billion to education funding.

Arizona Gov. Doug Ducey, a Republican, has proposed a 19 percent raise over the next three years, which, combined with the one percent raise from last year, would total 20 percent by the start of the 2020 academic year.

The striking teachers, however, say they will stick to the demand for increased school funding as well.

“If we pass a 20 percent pay increase and people continue to strike, I don’t think that makes sense to citizens, to parents,” Ducey said, according to the Arizona Republic. “I don’t think that makes sense to teachers.”

State Superintendent Diane Douglas told teachers, “Get back in the classrooms because your duty is to the students and parents.” She noted walkouts are illegal in Arizona, and the consequences for teacher classroom absences are unclear.