Researchers often presume that it is better to use administrative data from grades 4 and 5 than data from grades 6 through 8 for conducting research on teacher effectiveness that uses value-added models because (1) elementary school teachers teach all subjects to their students in self-contained classrooms and (2) classrooms are more homogenous at the elementary school level. We examined the first issue by using data on teacher-student links in which teachers of mathematics and/or English/language arts had verified the subjects and students they taught. We compared these data to teacher-student links from the original administrative data. Results show that instruction is often departmentalized in these grades. About one in six elementary school teachers in the original data was linked to a subject that he or she did not teach. To examine the second issue, we computed the variation in baseline student achievement within classes, between classes at the same school, and between schools. We found more within-school variation in pretest scores in middle school grades but an offsetting amount of between-school variation in upper elementary grades.