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Bernie Sanders today is announcing the foundation of his K-12 education plan, which is to crack down on public charter schools. If enacted, the Sanders plan would snuff out one of the most successful social policy innovations in decades, and close off a lifeline of opportunity for hundreds of thousands of poor urban children.

The charter-school sector varies enormously from state to state, but on average, charter schools yield better outcomes for urban students (though not for other students). States with the worst-regulated systems fare no better than traditional neighborhood schools. But the best-managed charter systems produce dramatically better outcomes for low-income urban children than the same students receive in neighborhood schools.

The public charter models with the highest success are the most exciting and deserving of replication. Rather than learn what they’re doing right, Sanders would choke them off.

One of Sanders’s splashiest ideas is a good one: He would ban for-profit charter schools, a move Democrats also endorsed in 2016. More than 80 percent of charter-school students attend nonprofit schools, and the for-profit schools perform significantly worse. A complete ban on for-profit charters is a blunt tool, but probably a positive one. Ending for-profit charters would incidentally take away one of the most effective arguments against charters. Charter opponents have erroneously convinced many people that all charters are for profit, when in fact only a small minority are. (Even some professional columnists have repeated this myth.) The charter sector would be stronger if the for-profit segment disappeared.

But Sanders’s goal is not to make charters better. Indeed, nothing in his plan pays any attention to what charter methods work best. Instead his plan reflects the perspective of the teachers unions, which despise charters because they avoid the hiring and pay-scale contracts that most unions rely on. Rather than pay teachers based on seniority, requiring the most recently hired teachers be fired first, and making it all but impossible to fire ineffective teachers, charters allow schools to pay teachers based on performance and replace the ones who can’t do a good job.

Sanders would “call for a moratorium on the funding of all public charter school expansion until a national audit on the schools has been completed” and “halt the use of public funds to underwrite all new charter schools if he is elected president.” He would additionally require charter schools to match employment practices with neighboring schools, meaning they would have to replicate the same rigid contracts, eliminating one of the key innovations that lets charters do a better job of teaching poor children.

Sanders’s plan appeals to a massive and highly organized Democratic constituency. But it does risk alienating African-Americans, who account for 26 percent of all charter students. A poll by Democrats for Education Reform found that while Democrats overall oppose charter schools, the racial divide is stark. Black and Hispanic Democrats favor charters — not surprisingly, given that their children benefit from them disproportionately — while white Democrats oppose them overwhelmingly:

Sanders frames his opposition to charter schools as a blow against segregation. But charter schools neither caused nor aggravated school segregation. School segregation is the result of residential segregation, and the system of neighborhood-based schools defended by the unions does nothing to break it down, either. As a way to ward off accountability and reform from the federal government, unions have increasingly emphasized “local control.” It is incredible to see opponents of charter schools simultaneously position themselves as somehow opposing segregation while also repeating the historic rallying cry of segregationists.

Neither traditional neighborhood schools nor charters do anything to reduce segregation. What charters do instead is offer poor urban children a better education. And the results at the best models are remarkable. No-excuses charter schools eliminate the achievement gap between white and black children. This is a staggering triumph of progressive social policy that should be spread and emulated.

A new paper by Sarah Cohodes, Elizabeth Setren, and Christopher R. Walters studies the charter sector in Boston, which is one of the most successful in the country. The Boston charter system, like many successful charters, presents a clear example for studies, because it has a hard cap on attendance that requires students be admitted by lottery. The lottery gives researchers a chance to measure the difference in results between students who win and get to enroll in a charter, and those who don’t. The charters do a far better job than the neighborhood Boston schools. Charter schools using these innovations have also produced lower teen pregnancy rates and higher four-year-college attendance.

It finds that the expansion of charters in 2010 did not dilute the enormous achievement gains by poor minority children. Rather, the schools managed to replicate the methods that enabled them to develop the potential in these students, including “high expectations, strict discipline, increased time in school, frequent teacher feedback, high-intensity tutoring, and data-driven instruction.” One of the major questions hovering over the charter movement is whether its extraordinary successes for underprivileged children can be scaled up. This important paper shows that they can, if politicians allow them to.

Rather than finding ways to replicate these successes, Sanders proposes to snuff them out. There are ways to appeal to teachers without attacking charters – Kamala Harris is doing this, by running on a huge teacher pay hike – but Sanders is instead standing athwart the reform movement yelling “Stop!” Given that teacher unions are more mobilized than parents, this might win Sanders more votes than it costs. The losers are poor students who will lose the chance to learn as much as their wealthy white counterparts.

Update: Kevin Drum objects that studies showing how high-performing charters produce enormous learning gains for low-income urban students all suffer a crippling flaw. They examine a population of students that applied for a charter lottery, so even though the lottery winners who attend a charter and the lottery losers who don’t may be identical, the winners and losers alike are unrepresentative because they all have parents who were motivated to apply for a charter. Maybe charters would not produce the same gains for kinds with unmotivated parents:

There is a difference between the charter kids and public school kids in general: the charter kids probably aren’t the very lowest performers, and at the very least they have parents who care enough about education to go through the whole process of applying to the charter. If the black charter winners do better than white charter losers, the most you can say is that the school has closed the black-white gap among kids who are at least moderately motivated.

This problem is a challenge for studying the effects of charters, but it’s not an insurmountable one, nor is it one education experts have failed to consider. Researchers have tried modelling the populations of students who apply and don’t apply, and one paper found that charters in Boston would actually produce larger gains by targeting students who are unlikely to apply. Drum’s thesis that charters are skimming off motivated students would predict that a massive expansion of the charter system would reduce the gains recorded by charters in Boston, but that did not happen. That’s why I cited the recent paper which finds that the gains in the Boston system can effectively scale up.

A Credo study models the populations in charters and non-charters, attempting to identify factors like parental involvement. This would also answer Drum’s objection. That study found significant gains for urban (but not suburban) students attending charter schools.

Statistical modelling is an indirect method. There are other ways to prove the same conclusion. In Denver and New Orleans, school systems have universal application systems, so there’s no difference between charter applicants and non-applicants. Both cities found big gains for charter students.

Any statistical effect can be quibbled with. The overall conclusion is very clear: a number of urban charter systems have produced vastly better educational outcomes for low-income urban minority students. Rolling back these transformative successes, as Sanders proposes to do, would be a heartbreaking historic error.