Across the country and in urban areas in particular, many students change schools during the academic year. While much research documents the impact of changing schools on the academic achievement of mobile students themselves, less research explores whether new arrivals have negative spillovers on stable classmates. The lack of research on impacts of mid-year entry is problematic, as poor, minority, and low-achieving students are disproportionately exposed to mid-year entry. In this study, we use a rigorous causal identification strategy and rich longitudinal data on fourth- through eighth-grade students in the New York City (NYC) public schools to estimate the impact of exposure to mid-year entry on the achievement of stable students. We analyze heterogeneous effects of mid-year entrants by origin (arriving from other NYC public schools, from other U.S. school systems, or from other countries), determine the extent to which mid-year entrants' characteristics mediate the impact of mid-year entry, and explore the moderating influence of stable students' characteristics. We find small negative effects of mid-year entry on both math and English language arts test scores in the short run. These impacts are not driven by mid-year entrant characteristics and are somewhat larger for Asian students and those who do not qualify for free or reduced-price lunch. Finally, results suggest mid-year entry continues to negatively influence the math performance of stable students beyond the year of exposure. We discuss the relevance of results and conclude with recommendations for future research.