The study of educational simulations at the secondary level has typically centered on programs that are competitive, shorter in duration, and characterized by their low fidelity, or a lack of realism. The resources required to hold longer, more immersive, and nuanced programs are often prohibitive for teachers of political science programs. As such, their effects on student learning outcomes have remained relatively unknown. In this paper, we explore the impact of a Model G20 (MG20) curriculum for high school and early college-aged students on students’ sense of self-efficacy and 21st century skills. MG20 is a weeklong, immersive international conference modeled after the real G20 summit. In it, students learn about global governance and roleplay as heads of state and government ministers to negotiate for their collective interests. Using a mixed methods approach, we examine student learning outcomes from two MG20 summits, held in the United States and in the UK. Results show that internationally diverse, immersive, collaborative role-playing simulations significantly improve students’ self-ascribed cross-cultural communication and public speaking skills, as well as students’ sense self-efficacy. This research suggests future study into new and emerging formats of educational simulations may reveal greater potential for such programs to enhance student learning.