Abstract Academic pressure in East Asia places students at heightened risk for diminished well-being, pointing to the need for interventions that sustain psychological functioning. This study investigated whether school adjustment moderates the effects of three character strengths interventions (signature, growth, and combined) on basic psychological needs satisfaction (autonomy, competence, relatedness) and subjective well-being. A quasi-experimental field trial was conducted with 549 fifth- and sixth-grade students in South Korea. Classrooms were assigned to one of the three character strengths conditions or an active control condition focused on emotions. Each programme included a classroom workshop and seven days of structured practice. Outcomes were measured pre- and post-intervention and analysed using multiple regression with interaction terms. At the overall level, character strengths interventions did not significantly differ from the active control. However, students with lower school adjustment specifically benefited significantly from the growth strengths intervention, reporting greater gains in autonomy and competence. These findings highlight the promise of growth strengths as a compensatory pathway for students struggling to adapt to school and stress the importance of culturally sensitive designs in high-pressure educational contexts. Trial registration: KCT0010992, retrospectively registered 9 September 2025.