The authors used a withdrawal design to evaluate an intervention designed to decrease interruptions during small-group instruction in a Kindergarten class. A new rule was put into place; during teacher-led small-group instruction, those not in the small group were required to address questions to designated student leaders, as opposed to interrupting the teacher. Results showed an immediate decrease in interruptions when the student-leader intervention was applied and an immediate increase when the intervention was withdrawn. Across-phase effect size analyses suggested large decreases in interruptions when the intervention was applied and a large decrease when it was withdrawn. The authors focus their discussion on identifying causal mechanisms responsible for intervention effectiveness and extending this research across grades levels, tasks, and target behaviors.