Just as students returned to school from their winter vacations, teachers in Los Angeles were ready to leave. Working in the second largest school district in the U.S., they were prepared to go on strike after failed negotiations in their quest for smaller class sizes, higher pay and increased funding for school counselors and nurses.

The Los Angeles dispute can be seen as a sign that the issues behind last year’s wave of walkouts—education cuts and low teacher pay, among them—aren’t resolved. But the story of teacher strikes in the U.S. goes back much further than that. Some historians have seen recent events as a resurgence of teacher activism that has roots in the 1960s and ’70s.

In the first half of the 20th century, the American Federation of Teachers and the National Education Association maintained no-strike policies. But as teachers faced the perpetual problems of difficult working conditions and lack of support for public education—and as conditions improved for other unionized professions—many chapters ignored that practice. Teacher strikes started to occur with greater frequency. More than 50 such strikes took place from 1946 to 1949, up from only a dozen in the first half of that decade, according to the Encyclopedia of U.S. Labor and Working-Class History.