Teachers at charter schools walked off the job Tuesday in Chicago — a first-of-its-kind strike in the charter sector. (Antonio Perez/Chicago Tribune/AP)

Teachers at a network of charter schools in Chicago went on strike Tuesday, a first for American charters and a sign they are facing issues similar to the traditional public schools they compete with.

More than 500 teachers and other staff members at 15 charter schools operated by the nonprofit Acero Schools walked out of the classroom after failing to reach a new contract. Issues in dispute include pay; class size, now set at 32 students; and the length of the school day and school year.

The educators are represented by the Chicago Teachers Union, which has organized about 25 percent of charter schools in the city. Nationally, about 11 percent of charters operate under collective bargaining agreements, according to the National Alliance for Public Charter Schools.

Charter schools are publicly funded but privately run, and they have drawn support from school-choice advocates such as Education Secretary Betsy DeVos, but also some Democrats. Nationally, nearly 7,000 charter schools serve about 3 million students.

For some backers, charters were seen as an alternative to traditional schools, free to innovate without the restrictions of union contracts and other district rules. That may no longer be the case, said Robert Bruno, a professor at the University of Illinois at Urbana-Champaign’s School of Labor and Employment Relations.

“The charter school movement is here to stay, but they’re no longer going to serve as this escape hatch for big-city mayors and school boards or investors or people who are critical of the public schools,” he said. “It isn’t going to be a low-wage workspace. It isn’t going to be a place where teachers don’t have a voice.”

The Chicago strike follows other efforts by teachers to improve their pay and working conditions, with walkouts this year in West Virginia, Oklahoma and other conservative states.

In California, teachers at the state’s largest virtual charter school, the for-profit California Virtual Academies, won a landmark union contract only after threatening to strike. A pact securing a pay raise and due-process rights was won from the school’s operator, education giant K12 Inc., after years of organizing and negotiations.

This year, a battle between unionized teachers and administrators at a charter school in the District broke into public view. Teachers at Chavez Prep Middle — the first D.C. charter school to unionize — argued that the school was spending millions of dollars on consultants while cutting core classroom positions.

In Chicago, the parties spent much of Tuesday negotiating.

“We will be on the picket line until they come back with an offer that respects our students and the people who educate them,” said Jesse Sharkey, president of the Chicago Teachers Union.

Richard L. Rodriguez, chief executive of Acero Schools, said the company has tried to reach a fair agreement maintaining policies that produce strong educational outcomes. “We are very disappointed that union leaders have put their anti-charter political agenda ahead of the interests of our students,” he said.

Among the issues is whether to maintain a longer school day and school year — the sort of policy that charter schools tout to families. The union calls that an imposition on staffers, who it says work 20 percent more than employees in the traditional school system.

Other issues include pay for teachers and paraprofessionals, and whether the contract should codify the schools’ policy of protecting undocumented immigrants.

The sides also are at odds over class size. The union said no teacher should be expected to manage a class of 32 students. The school network said there is no space in the buildings for additional classes, so smaller classes would mean fewer students served.

Randi Weingarten, president of the American Federation of Teachers, was in Chicago alongside striking workers. She said the walkout shows charters are “no longer in their infancy.”

“They’re in adolescence and, they like other public schools, if they want to be successful, they need to recruit and retain a stable teaching force,” she said.

Valerie Strauss contributed to this report.