Educators raise concerns about what happens to students when they are exposed to new teachers or teachers who are new to a school. These teachers face the challenge of preparing a year's worth of new material, perhaps in an unfamiliar work environment. However, even when teachers remain in the same school they can switch assignments--teaching either a different grade or a different subject than they have taught before. While there exists some quasi-experimental literature on the effects for student achievement of being new to the profession (e.g., Rockoff, 2004) or to a school (Hanushek & Rivkin, 2010), to date there is little evidence about how much within-school churn typically happens and how it affects students. We use longitudinal panel data from New York City from 1974 to 2010 to document the phenomenon, and we tie assignment-switching behaviors to available student achievement in the period since 1999.