Teachers at four Chicago International Charter School locations voted on Friday to authorize a possible strike, joining the Acero charter school teachers who voted in favor of a strike threat on Tuesday.

Of those who voted, 96 percent cast ballots to authorize a strike, according to the Chicago Teachers Union. The teachers voting work at CICS’ ChicagoQuest, Northtown, Wrightwood and Ralph Ellison campuses.

The CICS teachers have not announced a strike date, nor have the Acero teachers who are threatening to hit the picket line at 15 schools. If the strikes happen at all 19 schools, more than 700 teachers would walk off the job, affecting more than 8,000 students.

Like the Acero teachers, CICS teachers say they’re fighting for pay raises, smaller class sizes, improved special education resources and better wages for paraprofessionals. The union has slammed the charter operator for “bloated, wasteful management fees and salaries” that they say takes up one in every three dollars in funding received by CICS.

The starting salary for a CICS teacher is about $44,000, about $8,000 less than starting Chicago Public Schools teachers make. Civitas Education Partners, the management firm in charge of hiring and contract negotiations for CICS, says it agrees “our teachers should make more money” and that they’ve proposed a 4.4 percent salary increase.

“We believe in our teachers and staff and we believe that good-faith negotiations are an important opportunity to discuss and agree on issues that affect our students and their families,” Civitas admissions director Ulric Shannon said in a Friday statement. “We are committed to providing a fair and competitive wage and benefits package to our teachers and staff.”

Their contract expired in August. Both sides were back at the bargaining table Thursday, according to Civitas.

In a separate statement, CICS officials said they were “troubled by CTU’s threatened strike and the obvious harm it will cause to our students and their families,” but “hopeful” a teachers strike could be avoided.

Teachers have never gone on strike at any charter school in the United States. Work stoppages at such publicly funded, privately managed schools in Chicago have been narrowly avoided with last-minute agreements three times since 2016, including at Acero schools when the charter operator was known as UNO.