The study is titled The Secret Shame, a PDF file. I first saw reference to it in a John Hinderaker column, Liberalism Causes Achievement Gaps.
["The Secret Shame" author Chris] Stewart is a liberal activist from Minnesota who undertook to find out why the Twin Cities’ left-wing public schools have some of the country’s worst achievement gaps between white and minority (black and Indian) students.

Stewart compared achievement by race in a number of cities that he classified as progressive or conservative. The results didn’t surprise me, but they shocked Stewart. Conservative cities (as ranked by political scientists used as a reference for the study) consistently did a better job of closing student achievement gaps–sometimes, to zero–than progressive cities.
In a section of the study headed, "Of all the factors we looked at, progressivism is the greatest predictor," Stewart writes (page 12),
The scope of this report does not allow us to say what drives the correlation between progressive cities and underperformance of minority students. We did not consider any policy or practice as a cause for the larger achievement gaps between racial subgroups. But our results demonstrate that there is a negative difference between our most progressive and most conservative cities, and it can’t be explained away by factors such as city size, racial demographics, spending, poverty or income inequality. In cities where most of the residents identify as political progressives, educational outcomes for marginalized children lag at a greater rate than other cities.
Indeed, he would have been better off to leave out the page headed, "Recommendations" (page 29). He has none of any substance. He recommends getting everyone involved talking, "make better plans" (and "sign a petition for your city leaders to create a plan to help all children succeed in school"), "share information," and "make your voice heard."

Maybe in such community organizing blather he reveals his own progressive roots.

Having myself been progressive-free for many decades (I have the coins), I have a simpler analysis: Progressivism finds fault with the great setting, e.g. the greedy corporation, the racist society, or even world poverty. That's the focus of their reform efforts. Conservatism finds responsibility in the individual, and expectations are correspondingly focused on values and behavior, things like effort and persistence.

Dammit, it's a hard sell, though. How much easier is it to say, "Relax, reparations are coming." Just vote for easy street.

By the way, this study doesn't focus on charter schools at all.