Considerable evidence points to stress and health risks among students and teachers in modern schools. In recent years, mindfulness-based interventions have emerged as an answer to this growing strain. The present study implemented and evaluated a dual approach that introduced mindfulness simultaneously to students and teachers in three different German high schools. We investigated hypothesized improvements in areas of mental health, social-emotional competencies, and creativity among participants who engaged in a Mindfulness-Based Stress Reduction Course (MBSR) as well as the processes and mechanisms that underlie the potential effects. The study employed a controlled waitlist design across three schools. A total of 81 students (grade 11, mean age 16; 63 female, 18 male) and 90 teachers (mean age 45; 66 female, 24 male) participated in this study. In a mixed-methods approach, participants completed psychometric assessment (pre, post, and follow-up measurements) and qualitative semi-structured interviews. Among students, group comparisons revealed significant improvements with small to medium effect sizes on self-reported mindfulness, perceived stress, anxiety, depression, self-regulation, and emotional competencies. By contrast, teachers showed a significant improvement in medium effect size only on self-reported mindfulness. Explorative analyses and qualitative results expand teachers’ findings and point to benefits in stress management and social emotional competencies. Qualitative results further shed light on course mechanisms and processes among teachers and students. Results indicate a clearer benefit among students, yet also point to the supportive role mindfulness can assume among teachers. Systemic and implementation factors significantly shaped course results and experience in both populations.