In 1800, 90 percent of American schoolteachers were men; by 1900, three-quarters were women. The feminization of teaching—a job once filled primarily by transient young men, often saving up to finance a legal or medical education—was, in large part, why education became one of the few white-collar unionized professions in the United States.
The inescapable reality, however, was that schools were expensive, and Americans, then as now, didn’t like high taxes. So in order to rapidly open many more schools, states, cities, and towns made the conscious choice to hire mostly female teachers, who were cheaper to employ."
— ”The Chicago Strike and the History of American Teachers’ Unions” by Dana Goldstein