Charter schools are essentially less regulated public schools, free for students and free from unions’ and districts’ hiring requirements as well as most curricular constraints. They offer a popular alternative path to families in low-income districts where flagging reform efforts do less good than intended.

It’s a controversial format, subject to abuse and encouraging of pedagogical experimentation. Crowded waitlists testify to their desirability as well as to the negative view many parents hold of standard public schools. Forty-three states and the District of Columbia have some form of legislation on the books to support charter schools. But Massachusetts, Connecticut, and Washington state could soon be jamming the brakes.