Poor schools are getting poorer, while the rich get richer, a new study from Duke University finds.

“The average imbalance by race in North Carolina hasn’t really changed,” says Charles Clotfelter, a public policy professor who helped conduct the study. “North Carolina schools are becoming more imbalanced by economics than race. That’s something to worry about.”

It’s something to worry about because, as Clotfelter notes, schools with high percentages of low-income students find it hard to recruit and retain good teachers.

That’s a problem for low-income students, who usually perform worse on tests than their affluent peers, because highly qualified teachers have been shown to be one of the most effective tools for improving the performance of at-risk children.

Essentially, good teachers gravitate toward affluent schools, so creating high-poverty schools means setting yourself up for a dearth of good teachers in the classrooms that need them most. And, apparently, that’s what is happening across the state.

Clotfelter and a team of other researchers in Duke’s Sanford School of Public Policy studied all 100 counties in North Carolina going back to 1994. They found that racial balance had stabilized since the mid-2000s, which ended a trend of schools becoming more racially segregated.

Socioeconomic data showed the reverse trend. The state’s two largest school districts, Wake County and Charlotte-Mecklenberg, showed a particularly sharp divide.

Since 1994, CMS schools have become economically segregated at a rate nearly four times greater than Wake.

The comparison between Wake and CMS is particularly apt, because Charlotte and Wake both used to bus students to achieve socioeconomic diversity. CMS ended its policy in 2002, which has led to more neighborhood-centric schools.

Wake’s policy ended in 2010 at the hands of a Republican majority, which had just swept into power. However, the policy was still in effect in 2010-11, the last year studied by researchers.

Wake may also move back toward a diversity model now that Democrats have regained control of the Board of Education.

The researchers rated socioeconomic balance by how much individual schools’ poverty percentages lined up with the countywide percentage. Free and reduced lunch was used as the indicator for socioeconomic status.

Each school district was given a number to denote its level of imbalance—the larger the number, the more economically segregated the district. CMS scored 0.38 on the scale in 2010-11, while Wake scored 0.13. In 1994-95 the school systems had a virtually identical rate of imbalance. CMS scored 0.12 and Wake scored 0.08.

Sounding the effects of diversity in the classroom often leads to mixed conclusions. Research does not conclusively indicate that socioeconomic diversity automatically leads to better performance for low-income students, who tend to perform far worse than their affluent peers.

However, diversity does lay a good groundwork for success, given that it is difficult to retain and recruit high-quality teachers in poor schools. The conservative counterargument is often to channel additional money into high-poverty schools. Research has shown this can be effective, though liberal policy advocates tend to argue that it’s unsustainable.

Vance County, which sits along the Virginia border, is the most economically segregated school district in the state, the study concludes, with a rating of 0.46. CMS was second highest with its 0.38 rating. Chatham County schools were the seventh most economically segregated district with a score of 0.29. Durham County schools tied for that spot. Orange County schools were well down the list with a score of 0.08.

Here’s a link to the full study and one to the executive summary.