Teacher turnover itself doesn't adversely affect the quality of instruction - loss of experience resulting does

It is widely believed that teacher turnover adversely affects the quality of instruction in urban schools serving predominantly disadvantaged children, and a growing body of research investigates various components of turnover effects. The evidence at first seems contradictory, as the quality of instruction appears to decline following turnover despite the fact that most work shows higher attrition for less effective teachers. This raises concerns that confounding factors bias estimates of transition differences in teacher effectiveness, the adverse effects of turnover or both.

After taking more extensive steps to account for nonrandom sorting of students into classrooms and endogenous teacher exits and grade-switching, this study replicates existing findings of adverse selection out of schools and negative effects of turnover in lower-achievement schools.

However. the study finds that these turnover effects can be fully accounted for by the resulting loss in experience and productivity loss following the reallocation of some incumbent teachers to different grades.