Teachers’ strikes have been a constant across the country in 2018, popping up in six states, from West Virginia to Oklahoma. But so far, the wave of activism has been limited to educators at traditional public schools. That is, until earlier this week, when unionized teachers from one of Chicago’s largest charter-school networks, Acero Schools, took to the picket line. The strike is the first of its kind in U.S. history; although other charter-school teachers have unionized—collective bargaining is a requirement for charters in Hawaii and Maryland—these teachers are the first in the country to actually stage a walkout.

Teachers from all 15 Acero schools —which range from elementary to high school—resorted to a strike after failing to arrive at an agreement with Acero on Monday night over the terms of a new contract, in which they had requested higher salaries and smaller class sizes.

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While teachers’ unions such as the American Federation of Teachers (AFT) had been gearing up for a possible strike for weeks , this decision to walk out is unprecedented: For decades, charter schools have championed themselves as a radical experiment in education that eschews the constraints of unions and traditional-public-school bureaucracy in favor of the flexibility to innovate. Now, at least in Chicago, the mentality seems to be shifting—and charter-school advocates fear it’s a sign that teachers’ unions, in a desperate effort to retain their clout, are co-opting charters for their own political reasons. Concurrently, the head of Chicago Public Schools, Janice Jackson, recommended on Monday that the city stop accepting proposals for new charter schools, a moratorium also supported by J. B. Pritzker, the governor-elect of Illinois.