Fifteen Chicago charter schools are closed Tuesday after their teachers called a strike — making history with the nation’s first-ever strike of a charter operator.

After bargaining late into the night, teachers at schools in the Acero network decided early Tuesday morning to strike. They had authorized the strike in October over issues that include pay, teacher diversity, and class size.

The strike affects more than 7,000 children, whose parents were advised to keep their children home or find child care. The schools are also using non-union staff to watch children, network officials said.

Outside one of the glass-walled schools that is a crown jewel in the Acero architectural portfolio, dozens of teachers carried picket signs and participated in a robust call-and-response early Tuesday morning.

“They say cut back, we say fight back,” they chanted against the backdrop of a chilly grey morning. “Chicago is a union town.”

The strike represents the first major labor action by charter school educators unionized under the Chicago Teachers Union, which until recently represented only teachers in the city-run school district. Acero’s own teachers union merged with CTU, which represents Chicago public school teachers, last year amid a broad push by CTU to organize educators at publicly funded, privately run charter schools.

Since then, the union has been negotiating a contract for Acero educators and had set Monday night as a deadline to reach an agreement.

Read more: Why Chicago charter educators say now is moment for historic strike

Richard Rodriguez, Acero’s CEO, said in a statement posted on YouTube that network leaders were “disappointed” that the union “walked away from the bargaining table.”

“There is absolutely no good reason to put students and parents through the upheaval of a strike,” Rodriguez said. “Interests from outside our community are using our students and our schools as a means to advance their national anti-charter school platform.”

CTU’s charter unionization push is being closely watched by many in the education world. But Acero teachers said they were concerned first about their own students and schools.

Among teachers’ demands: higher pay, increased teacher diversity, smaller class sizes, and a shorter school year. Teachers said they hoped the changes would curb the network’s chronic staff turnover.

They are also asking for a clear policy that would prohibit immigration authorities from enforcing immigration law inside the schools, to protect their heavily Latino student population, say union officials.

Standing outside the network’s Veterans Memorial Campus Tuesday, special education teacher Kristin Maher said she was concerned about making sure that students with special needs get adequate support. At her school, Acero-Major Hector P. Garcia M.D. High School, she said, turnover is high among paraprofessionals who help students with disabilities.

“Although compensation is an issue, I am much more concerned about the funding they give special education,” said Maher, a special education teacher.

Maher’s colleague, Susana Urquiza, said she, too, was unable to meet her students’ needs. “I am the bilingual education teacher in a school of 175 students who are considered English learners and I am the only teacher in charge of the program,” Urquiza said. “Another teacher to help me with the program would be helpful. I’ve had to get subs for my classroom, so I don’t feel like I am really helping my students.”

Charter school teachers who walk out face steep stakes. Because they have private employers, any labor action they take is governed by the National Labor Relations Act, not the Illinois Labor Relations Act. That means they can be fired for taking collective action, while teachers employed by the school district can strike without risking losing their jobs.

Read more: 5 reasons to watch Chicago’s historic charter contract negotiations

Acero Schools, the charter operator formerly known as UNO, told Chalkbeat in November that it was committed to seeing negotiations through. “Based on statements CTU has made, there is a real focus on making an example out of charter schools,” the network said in a statement.

This is the second strike in the past decade overseen by the Chicago Teachers Union, which includes the Chicago Alliance of Charter Teachers and Staff. The CTU’s most recent strike, in 2012, targeted the entire Chicago Public Schools district and was largely seen as pushing through contract reforms that benefited teachers.

What happens during Acero’s strike could have implications beyond the network. The charter school bargaining unit is negotiating contracts with 11 operators this fall. Among them, teachers at Chicago International Charter Schools are negotiating contracts with two of the network’s management companies, and took a strike vote this fall. And the recent resignation of the founder of Noble, Chicago’s largest charter network, could open the door for its teachers to form the biggest charter union yet.

Acero and its teachers are returning to the table at 10 a.m. Tuesday.