This study examined whether the Incredible Years (IY) Teacher Classroom Management (TCM) program implemented as a school-wide preventive intervention at 1st to 3rd grade in a regular school setting reduces the development of problem behavior and improves social competence. Using a quasi-experimental pre-post design, the IYTCM was implemented in 21 schools and compared with 22 matched schools that did not receive the program. A total of 241 1st to 3rd grade teachers and 1518 students aged 6 to 8 years took part in the trial. Mixed-model analyses found small positive effects on changes in students' social competence (d[subscript w] = 0.19), while effects on change in students problem behavior were less than small (d[subscript w] < 0.20). When the program is implemented as school-wide universal preventive intervention, results suggest a small preventive impact of the IYTCM program in regular school settings for some of the outcomes measured in the study.