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 jobs—teaching either a different grade or a different subject than they have taught before. There is an extensive literature that explores the various aspects of the challenges confronting new teachers (see, for example, Feiman-Nemser, 2003; Johnson, 2007). 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. In this paper, we use longitudinal panel data from New York City from 1974 to 2010 to document the phenomenon, and we tie assignment-switching behaviors to student achievement in the period since 1999, when student-level data is available. We find that students are far more likely to have a teacher who has undergone a within-school switch than one who is new to the school or new to teaching. We therefore use a variety of fixed effects approaches to estimate the link between student achievement and these three forms of being to new one’s job assignment—new to teaching, new to school, or new to position within the same school—with a particular focus on the latter given that so many teachers experience within-school reassignments and we know so little about how students are affected by it.