Background <p>The rising rates in depression and other mental health problems among adolescents in recent years, combined with delayed help-seeking, highlights a clear need for action in key areas of adolescents’ lives. The school environment, including teachers, can provide mental-health support for children and adolescents.</p> Methods <p>The study used a pre-post-follow-up design to investigate whether a two-hour online training course on depression and mental health in childhood and adolescence leads to knowledge acquisition, confidence in supporting pupils with depression and a decrease in stigmatisation in <i>N</i> = 97 teachers, school social workers and school social pedagogues.</p> Results <p>We found a significant increase in knowledge and confidence in supporting pupils with depression at post-assessment, which was maintained at a four-week follow-up. A decrease in stigmatisation was found at post-assessment, which was no longer evident at follow-up.</p> Conclusion <p>The results suggest that approaching educational professionals through a concise practical online training course can be a promising strategy for knowledge transfer. Future studies could examine whether such trainings can be adapted to other mental health disorders.</p> <p><i>Trial registration</i> ClinicalTrials.gov, Identifier: NCT06387108. Registered on 24.04.24.</p>