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.