As positive psychology is a nascent area of research, there are very few empirical studies assessing the impact and sustained effects of positive psychology school interventions. The current study presents a 2-year longitudinal evaluation of the effects of a school-based positive psychology program on students' subjective well-being, school engagement, and academic achievements. The study investigated the effectiveness of the Maytiv school program using a positive psychology-based classroom-level intervention with 2517 seventh- to ninth-grade students in 70 classrooms, from six schools in the center of Israel. The classes were randomly assigned to intervention and control conditions, which were comparable in terms of students' age, gender, and socio-economic status. Hierarchical linear regression analyses revealed positive intervention effects on positive emotions, peer relations, emotional engagement in school, cognitive engagement, and grade point average scores (Cohen's ds 0.16–0.71). In the control group, there were significant decreases in positive emotions and cognitive engagement, and no significant changes in peer relations, emotional engagement or school achievements. These findings demonstrate the significant socio-emotional and academic benefits of incorporating components of positive psychology into school curricula.