Policy makers and analysts often view the reduction of student mobility across schools as a way to improve academic performance. Prior work indicates that children do worse in the year of a school move, but has been largely unsuccessful in isolating the causal effects of mobility. We use longitudinal data on students in New York City public elementary and middle schools to isolate the causal effects of school moves on student performance. We account for observed and time-invariant differences between movers and non-movers using rich data on student sociodemographic and education program characteristics and student fixed effects. To address the potential endogeneity of school moves arising from unobserved, time-varying factors, we use three sets of plausibly exogenous instruments for mobility: first-grade school grade span, grade span of zoned middle school, and building sale. We find that in the medium term, students making structural moves perform significantly "worse" in both English language arts (ELA) and math, whereas those making nonstructural moves experience a significant "increase" in ELA performance. In the short term, there is an additional negative effect for structural moves in ELA. These effects are meaningful in magnitude and results are robust to a variety of alternative specifications, instruments, and samples.