We conducted a potential efficacy trial examining the effects of classroom-wide implementation of the Pyramid Model for Promoting Young Children’s Social-Emotional Competence on teachers’ implementation of Pyramid Model practices and children’s social-emotional skills and challenging behavior. Participants were 40 preschool teachers and 494 children. Using a randomized controlled design, 20 teachers received a professional development (PD) intervention to support their implementation of the practices. The 20 teachers in the control condition received workshops after all study-related data were collected. Teachers who received PD significantly improved their implementation of Pyramid Model practices relative to control teachers. Children in intervention teachers’ classrooms were rated as having better social skills and fewer challenging behaviors relative to children in control teachers’ classrooms. Exploratory analyses showed that children at elevated risk for behavior disorders in intervention teachers’ classrooms had improvements in their observed social interaction skills relative to similar children in control teachers’ classrooms.