The study aimed to train teachers in managing student behaviour and to investigate the impact on teachers and their students. The training was based on adult-learning and group-leading strategies (development/application) as well as Social-Emotional-Learning and School-Wide-Positive-Behaviour-Supports approaches (content). It consisted of training meetings, coaching and distance learning and was implemented at Thessaloniki middle school, using a neighbouring middle school as the control school. The stability of impacts was checked by follow-up tests after four and twelve months accordingly. A convergent parallel mixed-methods design was used for data analysis. After completion of training and four months later, an effective behaviour-management methodology and corresponding strategies were developed in the intervention school. Teachers reported increased professional self-efficacy, teachers and students evaluated their school climate more positively and students' office discipline referrals decreased, compared with the control school. Benefits had decreased a year later, however, they remained increased compared with the control school. Ways to maintain beneficial results and a sustainable in-service teacher professional development policy are discussed.