Over the past 20 years, a rising tide of voices in the world of public policy has been telling us that public education has fallen into an abyss of mediocrity. Our schools are “broken,” the mantra goes. Principals and teachers — their lack of “rigor” and “low expectations” for their students — are the primary offenders. The problem can be “fixed” only if schools are held to strict accountability. “No excuses” are to be permitted.

The pressure intensified in 2002 with the enactment of the federal testing law No Child Left Behind, which mandated high-stakes standardized exams that were supposed to bring every child to “proficiency” by the year 2014. When it grew apparent that this goal would not be reached, privatizing leaders pounced, offering business-modeled interventions as, perhaps, the only viable solution. Prominent figures in financial circles and at large foundations became interested in charter schools, encouraged their expansion and provided grant support to some of them. Others, with less philanthropic motives, saw a market opportunity and started running charter schools for profit. What had been a slowly growing movement now became a juggernaut.

Diane Ravitch was for many years one of the strongest advocates for the testing-and-accountability agenda. Because of her impeccable credentials as a scholar and historian of education, she was a commanding presence among critics of our schools. Some years ago, however, she reconsidered her long-held beliefs and, in an influential book, “The Death and Life of the Great American School System,” parted ways with her former allies and joined the highly vocal opposition.

In her new book, “Reign of Error,” she arrows in more directly, and polemically, on the privatization movement, which she calls a “hoax” and a “danger” that has fed on the myth that schools are failing. Scores go up and down from year to year — usually, as she explains, because the testing instruments are changed and vary in their difficulty. But, pointing to the National Assessment of Education Progress, which has sampled math and reading scores every two years since 1992 and, in an alternate version, every four years since the early 1970s, Ravitch demonstrates that levels of achievement have been rising, incrementally but steadily, from one decade to the next. And — surprise! — those scores are now “at their highest point ever recorded.” Graduation rates are also at their highest level, with more young people entering college than at any time before.