Current school turnaround policies that require schools to replace no less than 50 percent of their staff offer an overly simplistic view of how to improve schools: take out the "bad teachers," insert the "good teachers" and presto, chango, out comes a better school! If only it were so simple. Evidence from a new study drawing on New York City data shows the reality is much more complex and highlights the potential downsides of teacher re-shuffling in the lowest performing schools.

The study goes beyond an analysis of how teacher turnover affects the achievement of students of "movers" and delves into whole-school effects. It finds that in low-income, high-minority schools, the negative effects of turnover reach beyond the classrooms of "movers" and into those of "stayers," even when controlling for teacher effectiveness. At these schools, students of teachers who merely had the staff around them change performed worse. The effect sizes were not huge, but big enough to be cause for worry, especially when considering that staff replacement was intended to help, not harm student achievement. While systematically replacing ineffective teachers with much more effective teachers no doubt leads to a net gain, these off-setting, school-wide impacts reinforce the need to make staff changes judiciously and with good data in hand.

Without knowing more about what causes the wider school impact, it's tough to mitigate. However the seemingly magical formula of school turnarounds fails to address the nuances of school communities and teaching. In doing so, it becomes counter-productive. This is a clear sign to reformers that school improvements must happen in a more holistic fashion. If not, students at our lowest performing schools may be left even further behind.