<p>The dynamic and diverse nature of international schools poses unique challenges and opportunities for implementing Social-Emotional Learning (SEL) programs. SEL can support international students by teaching them skills that may help mitigate the negative effects of frequent school transitions and reduce stress-related mental health problems. This study investigates the implementation of the SEL program <i>Fly Five</i> in international schools in Morocco. Specifically, it examines the program uptake, cost of implementation, and its influence on relevant SEL outcomes during the first two years. Data collection occurred at three sites (two intervention and one comparison school), involving 42 classes across grades K–8 (<i>N</i> = 311; 49.4% female; M_age = 11.3 years). Multilevel analyses revealed that, by the end of Year 1, students in the intervention group demonstrated greater improvements in SEL competencies relative to the comparison group (<i>ES</i> = 0.47), with the largest gains observed in relationship skills and social awareness. However, no additional improvements in SEL competencies were observed at the end of Year 2 or in students’ mental health outcomes. Interviews and classroom observations indicated that buy-in and fidelity of implementation were higher during the first year and in the elementary grades. The cost analysis highlighted heterogeneity in program implementation and explored how costs varied by teaching time. Findings provide new insights into the implementation of SEL programs in international schools and other diverse school settings, highlighting factors affecting their success, such as longer teaching time, comprehensive teacher training, and lower student age.</p>