The magnitude of student mobility was examined in the Texas public schools by reporting how many students are moving, when and where they are moving, and who is moving, and by clarifying the relationships between mobility and academic performance at the individual student, campus, and district levels. While the study's primary focus was on within-year student mobility, the issues also were examined longitudinally by following the Grade-1 students of 1991-92 through the 1995-96 school year. Analyzing data from the Texas Public Education Information Management System and the files of the Texas Assessment of Academic Skills led to the conclusion that there are significant relationships between student mobility, academic performance, and school accountability. Mobile students scored lower on the state-required tests than students who did not move, and this negative relationship became even stronger in schools with high student turnover rates or percentages of economically disadvantaged students. Student turnover rates also were negatively related to both campus and district accountability ratings. Recommendations and future research needs are discussed. Appendixes present the analysis category descriptions and five statistical summary tables. (Contains 9 figures, 14 tables, and 33 references.) (Author/SLD)