|Between one fall and the next, nearly forty percent of students change schools: some due to the grade configuration (e.g., structural moves from middle to high school), others are voluntary over the summer between academic years (nonstructural between year), and others occur during the school year (nonstructural within year). We use administrative data from Tennessee to test whether observable characteristics predict each of these moves, relative to staying at the school, and estimate the effect of these moves on academic achievement. We find that economically disadvantaged, underrepresented minority students, and students with a history of suspensions, prior mobility, and chronic absenteeism are more likely to change schools nonstructurally between school years and within school years. We then estimate that changing schools nonstructurally within year results in a -0.09 SDU loss in math achievement gains, or the equivalent of 60 days of school, while between year nonstructural moves have a small, positive effect on math gains. Finally, we perform a formal mediation analysis using novel measures of changes in school quality, assimilation, and disruption. We find evidence that benefits to between year nonstructural moves are caused by moving to destination schools with higher value added estimates than the origin school; while assimilation accounts for 70% of the negative causal effect of within year nonstructural mobility and disruptions in learning exacerbates the negative effects of within year nonstructural mobility. Implications for policy and practice are discussed.