Background Students in the United States spend a meaningful portion of their developmental lives in school. In recent years, researchers and educators have begun to focus explicitly on social and emotional learning (SEL) in the school setting. Initial evidence from meta-analyses suggests that curricula designed to promote SEL likely produce benefits in terms of social-emotional competence (SEC) and numerous related behavioral and affective outcomes. At the same time, there are often barriers to implementing such curricula as intended, and some researchers have questioned the strength of the evaluation data from SEL programs. As part of the effort to improve programming in SEL, this paper describes the protocol for a cluster randomized trial of the ACT OUT! Social Issue Theater program, a brief psychodramatic intervention to build SEC and reduce bullying behavior in students. Objective The objective of this trial is to examine if a short dose of interactive psychodrama can affect SEC metrics and bullying experiences in schoolchildren in either the short (2-week) or medium (6-month) term. Methods The ACT OUT! trial is a cluster randomized superiority trial with 2 parallel groups. The unit of measurement is the student, and the unit of randomization is the classroom. For each grade (fourth, seventh, and 10th), an even number of classrooms will be selected from each school—half will be assigned to the intervention arm and half will be assigned to the control arm. The intervention will consist of 3 moderated psychodramatic performances by trained actors, and the control condition will be the usual school day. Outcome data will be collected at baseline (preintervention), 2-week postintervention (short term), and 6-month postintervention (medium term). Outcomes will include social-emotional competency; self-reported bullying and experiences of being bullied; receptivity to the program; and school-level data on truancy, absenteeism, and referrals to school displinary action for bullying. A power analysis adjusted for clustering effect, design effect, and potential attrition yielded a need for approximately 1594 students, consisting of an estimated 80 classrooms split evenly into intervention and control arms. Results This study was funded in June 2019; approved by the Indiana University Institutional review board on September 17, 2019; began subject recruitment on November 5, 2019; and prospectively registered with ClinicalTrials.gov. Conclusions Many states have issued recommendations for the integration of SEL into schools. The proposed study uses a rigorous methodology to determine if the ACT OUT! psychodramatic intervention is a cost-effective means of bolstering SEC and reducing bullying incidence in schools. Trial Registration ClinicalTrials.gov NCT04097496; https://clinicaltrials.gov/ct2/show/NCT04097496 International Registered Report Identifier (IRRID) PRR1-10.2196/17900