Student mobility that occurs within a school year may be especially disruptive for student outcomes, yet little is known regarding the predictors of within-year mobility. In particular, research has yet to comprehensively examine the role of student achievement in predicting within-year student mobility. Thus, we sought to understand this link by examining longitudinal 3rd-8th grade student- and school-level data. We conducted (1) random effect panel regression models to consider the ways in which student achievement in math and English and language arts (ELA) predict within-year student mobility and (2) survival models to examine these mobility patterns over time. We found that achievement levels were significantly associated with student mobility and that low achievement in 3rd grade had lasting effects on student mobility. We close with implications for policy and practice.