The current study investigated the effects of school mobility on the academic achievement of different racial/ethnic groups in four cohorts of students from a very large urban school district. In this study, I compared within-year and between-year mobility and, most importantly, account for all the schools students attended over the study period. Using a multiple membership model (MMM), the findings confirmed that, for all student groups, academic achievement was affected more by within-year school mobility than between-year school mobility. Black students had the highest mobility rates, both for between- and within-year mobility. Although Asian-American students achieved higher reading and math scores on average, they were more negatively impacted by within-year school mobility compared to other groups. The current study was able to pinpoint the students most at risk for negative outcomes following within-year mobility. The findings are discussed in the context of policy recommendations that can be adopted by school districts.