“The focus of the discussion today is maintaining our own identity as a network of schools,” she said. The promise of an extended school day and year is “a real differentiator and important to our families.”

The strike comes shortly after a midterm election in which a burst of progressive Democratic energy led to defeats for charter school advocates in Illinois and other states.

That teachers in a charter network were able to organize, let alone walk out of their classrooms en masse, is notable given the history and aims of charter schools.

Only about 11 percent of the nation’s 7,000 charter schools are unionized, according to the National Alliance for Public Charter Schools.

Although the union leader Al Shanker helped popularize the concept of charter schools in the 1980s, intending them as laboratories for educational innovation, he became a fierce critic when reformers began using charter laws to open nonunion schools. The reformers hoped that student performance would improve outside the bureaucratic constraints of contract work rules, and that underperforming teachers could be fired more easily. Results in the charter sector have been mixed. At Acero schools, a quarter of students met standards on state exams in 2016, the same percentage who met standards in the Chicago district schools, according to a report from the Illinois State Board of Education.

As the national charter sector grew rapidly, now serving over 3 million children, it developed a complicated relationship with organized labor. Union leaders accused charters of leeching students and funding from traditional public schools, and tended to oppose the expansion of the sector. But the unions also sought to organize charter schoolteachers and, in some cases, even opened their own charter schools.

When it comes to organizing the charter sector, unions “have goals that are in some ways in contradiction” to one another, said Daniel DiSalvo, a professor of political science at the City University of New York and a fellow at the Manhattan Institute. “They want to win better things for the individual teachers, but fight back against the system they’re participating in.”