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 not reaching 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.”