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 first grade classrooms. "INSIGHTS" is a temperament-based social- emotional learning intervention with teacher, parent, and classroom programs. Twenty-two 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. Multi-level modeling demonstrated larger impacts of "INSIGHTS" on instructional support in first grade classrooms with greater mean classroom-level shyness scores. A further set of multi-level analyses showed larger impacts of "INSIGHTS" on math skills for students in classrooms with greater mean classroom-level shyness scores. Practice and Policy: Results suggest the importance of considering temperament at the classroom-level when deciding how to allocate limited resources to 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. [This paper was published in "Early Education and Development" 2018.]