Here is a very good look (pdf) at the teacher equity problem from Tennessee (and it’s a problem most places, they just have better data than most):
The report has serious implications for education policies designed to match effective teachers with the students who need them the most. It finds a clear equity problem. Low-income and minority children have the least access to the state’s most effective teachers and more access to the state’s least effective teachers. Although many of the beginning teachers in high poverty/high minority schools are among the state’s most effective, many of them do not stay in these schools or lose their effectiveness over time.