Student-teacher interactions are dynamic relationships that change and evolve over the course of a school year. Measuring classroom quality through observations that focus on these interactions presents challenges when observations are conducted throughout the school year. Variability in observed scores could reflect true changes in the quality of student-teacher interaction or simply reflect measurement error. Classroom observation protocols should be designed to minimize measurement error while allowing measureable changes in the construct of interest. Treating occasions as fixed multivariate outcomes allows true changes to be separated from random measurement error. These outcomes may also be summarized through trend score composites to reflect different types of growth over the school year. We demonstrate the use of multivariate generalizability theory to estimate reliability for trend score composites, and we compare the results to traditional methods of analysis. Reliability estimates computed for average, linear, quadratic, and cubic trend scores from 118 classrooms participating in the MyTeachingPartner study indicate that universe scores account for between 57% and 88% of observed score variance. (Contains 7 tables, 1 figure and 2 footnotes.)