The authors investigated the relationship between student mobility and student performance and behavior. The authors used criterion-referenced test (CRT) and norm-referenced test (NRT) data indexes from the 1998-1999 school year. Results showed that as the mobility of students increased within the school year, their test performance on the CRT and the NRT decreased. Also, suspension rates were high for students who had changed schools within a school year. As a practical solution, students who experience single or multiple transfers within a school year should receive particular attention because they are likely to have discipline and performance problems. Also, the K-12 grade structure appears to be much more appropriate for students than is the traditional K-5, 6-8, and 9-12 structure. (Contains 19 notes and 19 tables.)