Social skills are dynamic developmental constructs typically measured using assessments developed via cross-sectional methods. The measurement model of derivatives (MMOD), a factor analytic approach targeting individual growth trajectories, was used to evaluate the longitudinal factor structure of the Social Skills Improvement System--Rating Scales (SSIS-RS) teacher form with a sample of 1320 first and second grade students (51.6% female, 54.8% white, 20.6% Black, 14.4% Hispanic). Although results provided support for three of the original SSIS-RS factors (Assertion, Empathy, Self-Control), there was some evidence that the other four original domains (Cooperation, Responsibility, Engagement, Communication) could be explained by two factors in this age range. Implications for using the SSIS-RS and the utility of the MMOD in developmental research are discussed.