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 (BAU) or intervention, which included delivery of 34 group and 12 individual recess coaching sessions over 6 months, teacher consultation and inservice 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. Results indicated few significant main effects, with consistently small effect sizes. Effects were generally larger for self-regulation and oppositional defiant disorder (ODD) outcomes. For students at-risk for ODD, several additional effects were observed at follow up including on emotion regulation (d = 0.75) and academic proficiency (d = 0.55). 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. Results contribute to understanding transportability and have useful implications for school mental health programming. [This is the pre-print version of an article published in "School Mental Health."]