It's well documented that schools serving low income and minority students have greater teacher turnover and more rookie teachers. Teachers appear to prefer to work close to home and they can only do so once they have more experience in the district, when they can take advantage of seniority provisions to get their pick of assignments.

However, a new study by Cornell economist C. Kirabo Jackson suggests there may be more at play here than simply teachers' desire for a shorter commute.

Jackson took advantage of Charlotte-Mecklenberg's 2002 decision to end its decades-long student busing program (a program which had kept the district fairly well integrated), examining in more detail teachers' decisions about where they work. He found that schools with a new influx of black students generally experienced a decrease in their share of high-quality teachers, as measured by years of experience and certification test scores. Teacher effectiveness in terms of student learning gains also declined. In other words, the best teachers (both black and white) were more likely to transfer schools, though black teachers on average were more likely to stay in their assignment than white teachers. However, the black teachers who stayed were not as strong as the black teachers who left.

It's a disquieting finding that teachers sort themselves in the face of anticipated changes in student race. The biggest challenge for districts is how to remedy the resulting disparities in teacher effectiveness. Last year when Charlotte-Mecklenberg offered its best teachers a hefty $10,000 bonus and 15 percent pay raise if they transferred into a school with higher minority populations, only 30 teachers agreed to make the switch.