The current study evaluated the effectiveness of a training program for inservice secondary school teachers in classroom management. In a non-randomized pre-post-design, 19 teachers participated in a newly developed training (the intervention group) and 18 teachers participated in a control training (the control group). All participants reported better knowledge of classroom management after training. However, hypothesized positive effects on teachers' competencies and increased student engagement occurred only in the intervention group. These findings are supported by participants' reported high subjective validity of the training. Further research is needed to study sustainability of the observed effects. (Contains 3 tables and 1 figure.)