School-based social and emotional learning programs aim to provide students with the skills they need to deal with life challenges, thereby enhancing their social and emotional wellbeing, academic outcomes, and reducing their risk of mental health difficulties. While there is a robust evidence base on the effectiveness of these programs originating from the US, there is a relative paucity of research on how these programs impact young people in other county contexts, especially for older adolescents and those at higher risk. This study sets out to address this research gap by evaluating the effectiveness of a social emotional learning program designed for older adolescents in Ireland, the MindOut program. MindOut is a universal school-based social and emotional learning program designed for older adolescents in Ireland which was developed based on a common elements approach underpinned by CASEL’s framework. Employing a cluster randomized-controlled trial, data on social and emotional skills, academic performance and mental health outcomes were collected from students (n = 497; 51.1% female) ages 15–18 years in 32 disadvantaged schools. There were significant improvements in intervention students’ social and emotional skills including, reduced suppression of emotions (p = 0.035), use of more positive coping strategies [reduced avoidance coping p = < 0.001) and increased social support coping p = 0.044)]. Improvements in mental health and wellbeing were also found with significantly reduced levels of stress (p = 0.017) and depressive symptoms (p = 0.030) as well as reduced anxiety scores for females students (p = 0.044). These short-term evaluation findings support the positive impact of school-based social and emotional learning programs, such as MindOut, when designed to be both age and culturally appropriate and delivered to older adolescents in disadvantaged schools.