Research Findings: This study investigated how mean classroom-level shyness scores moderated the impacts of "INSIGHTS" into "Children's Temperament" on instructional support and students' math and reading skills in kindergarten and 1st-grade classrooms. "INSIGHTS" is a temperament-based social-emotional learning intervention with teacher, parent, and classroom programs. A total of 22 low-income urban elementary schools, 90 teachers, and 435 children were included in the study. Schools were randomly assigned to "INSIGHTS" or an attention-control condition. Multilevel modeling demonstrated larger impacts of "INSIGHTS" on instructional support in 1st-grade classrooms with greater mean classroom-level shyness scores. A further set of multilevel analyses showed larger impacts of "INSIGHTS" on math skills for students in classrooms with greater mean classroom-level shyness scores. Practice or Policy: Results suggest the importance of considering temperament at the classroom level when deciding how to allocate limited resources to the implementation of temperament-based intervention and/or social-emotional learning programs. Moreover, providing professional development supports to teachers of shy students--who are at risk for poorer instruction and academic skills--should be considered by policymakers and practitioners.