Brill obliquely refers to such research in half a sentence. He then counters that other studies have shown better results for charters, without clearly indicating what these studies are or explaining why they should trump a comprehensive, national study. He then points to the “central evidentiary value” of the Knowledge is Power Program, KIPP, the chain of roughly 100 charter schools, founded by two Teach for America alumni, that has produced consistently high student test scores and become a media darling. Yet such exceptions to the rule still don’t explain why, if unions are the crucial variable, a vast majority of charters haven’t equally thrived.

At the heart of Brill’s book is a belief that “truly effective teaching” can “overcome student indifference, parental disengagement and poverty.” For too long, Brill’s reformers argue, union leaders have used such factors to excuse failing teachers protected by tenure. Certainly many adults, not just those in unions, have written off economically disadvantaged or minority students far too readily.

Brill cites policy advocates who argue that students who have top quartile teachers several years in a row could (at least theoretically) make remarkable gains. Absent other proven criteria for determining the most effective teachers, these reformers conclude that schools should base hiring, firing and promotion decisions, at least in substantial part, on teachers’ ability to generate year-to-year gains on their students’ test scores.

Brill, however, glosses over an important qualifier to such research. Teacher quality may be the most important variable within schools, but mountains of data, going back decades, demonstrates that most of the variation in student performance is explained by nonschool factors: not just poverty, but also parental literacy (and whether parents read to their children), student health, frequent relocations, crime-­related stress and the like.

Brill extols the recent documentary “Waiting for ‘Superman,’ ” which argues that better teachers are the key to boosting achievement. But surprisingly, what we see in the movie aren’t so much good teachers as academically effective parents: mothers and fathers who, despite difficult circumstances, read with their children, push them to do their homework and actively seek out exceptional charters, which (unlike the mediocre or failing ones) are oversubscribed and thus rely on lotteries with long odds for admission.

Yet to Brill and the filmmakers, these parents’ love, sweat and tears must be irrelevant, because what really matters is the quality of a child’s teacher. To prove the point, Brill cites one study that shows that students who won the lottery subsequently performed better in school than those who lost. “Same demographics, same motivation, different result,” he concludes.

But this argument ignores the aggregate effect of student and parental attitudes. Children who don’t win a coveted spot at a program like KIPP don’t just miss the charter’s arguably better teachers; they also lose out on the self-­reinforcing atmosphere of success and striving that comes from attending a school where everyone — or at least most students and parents — has demonstrated an especially deep commitment to learning. At KIPP, for example, students go to school longer each day, each year, and also attend classes on alternating Saturdays and in the summers. Families that don’t embrace this ethos leave or can be asked to leave, an option not available to regular public schools.