It's not just experts but just good old common sense that tells us that a constant churn of teachers in and out of a school is disruptive.  But we also know that keeping ineffective teachers in the classroom in pursuit of stability does a disservice to their students.  Mark Simon (a DCPS parent, the former president of the Montgomery County teachers union and an education policy analyst at the Economic Policy Institute) took a look at the issue of teacher turnover and retention in DCPS and points the finger at the Rhee/Henderson reforms, calling them out for pushing effective teachers out of the classroom.  Repeating the claims of an independent budget consultant, he asserts that DCPS attrition rates for novice teachers are substantially higher than the national average.

This begs two questions:  First, is there any evidence that current DCPS attrition rates are any higher than what they were in the pre-Rhee era?  Second, are effective teachers voluntarily leaving or being asked to leave teaching disproportionally, as he suggests? 

We don't know the answer to the first question, but looking at DCPS data on retention (the flip side of attrition), the annual retention rate of teachers who were rated highly effective by the IMPACT teacher evaluation system is significantly higher than that for those rates as less effective.