On July 30, 2011, thousands of public school teachers rallied on the southwest corner of the Ellipse, near the White House. Union members mingled with the occasional communist pamphleteer, and, on a temporary stage, a series of activists, students, scholars, and teachers put forward variations on a theme: Standardized tests and corporate interests are ruining public education.

Late in the program, the actor Matt Damon showed up and began chatting amiably with an older, gray-haired woman sitting next to him on the stage. It turned out he wasn’t the only star in attendance. The next speaker “is the torchbearer, the champion for children,” an organizer announced. “Like Britney and Cher and Gaga, in the education world, all you need to say is ‘Diane.’ ”

The gray-haired woman walked to the microphone as the crowd chanted, “Diane! Diane!” “This is a historic day. I’m a historian,” she told them. She spoke for only eight minutes, in short, punchy sentences. “Carrots and sticks are for donkeys.” “Education is a right, not a race.” “Our problem is poverty, not our schools.” When she finished, the crowd began chanting again: “Thank you! Thank you!”

It was, historically speaking, a strange place for Diane Ravitch to be. There was no indication that, until recently, she had championed many of the policies that were denounced at the rally as tools of racism and oppression. That she had spent years in the inner circle of conservative education policy, advocating for school vouchers, firing incompetent teachers, and shutting down failing schools. Ravitch once assured the public, “Vouchers and charters will not destroy public education. This is an incredible and fantastical fear.” Now she says things like, “Vouchers are a con, intended to destroy public education.”

Improbably, at the end of a four-decade-long career as the nation’s most prominent education historian and a vocal advocate for education reform, Ravitch has...