Several mental health programs have been developed in clinics and transported into schools, which has great potential for increasing access to intervention for students who may not be otherwise served. However, such programs may lack consideration of the complexity and constraints of schools, including the diversity of student needs and backgrounds, raising questions about their effectiveness in this context. This study evaluates the efficacy an evidence-based clinical program—the Incredible Years® Dina Dinosaur School small group treatment program—under such realistic conditions. A total of 138 first- and second-grade students identified as having self-regulation difficulties were randomized to Business as Usual or intervention, which included delivery of 34 group and 12 individual recess coaching sessions over 6 months, teacher consultation and in-service presentations, and three parent workshops. Multi-method outcome measures were collected before and after the intervention and at 6-month follow-up, evaluating self-regulation, disruptive behavior, social competence, and academic proficiency. The results indicated few significant main effects and consistently small effect sizes. Effects were generally larger for self-regulation and oppositional defiant disorder (ODD) outcomes as well as for students at risk for ODD; however, meaningful improvement on functional school outcomes was limited. Findings suggest less impact than clinic-based delivery which typically includes a parent program component, although reduced effects may also be related to lower fidelity in some of the child groups. The results contribute to understanding transportability and have useful implications for school mental health programming.