This quasi-experimental pre-post comparison group design examined if the Incredible Years (IY) Teacher Classroom Management (TCM) programme implemented as a school-wide universal preventive intervention to students aged 6-8 years at the lower primary level in a regular school setting had an effect on teacher-student relationships and teacher-parent involvement. The IY-TCM training was delivered simultaneously to the entire group of school staff in first to third grade. Teacher reports in 21 intervention schools were compared to teacher reports in 23 control schools. A total of 241 teachers and 1,518 students took part in the trial. Linear mixed model analyses suggest modest positive effects on change in teacher-student closeness (d[subscript w] = 0.22) and conflict (d[subscript w] = 0.15), where a moderator analysis showed a significantly higher treatment effect for high-risk students on change in teacher-student conflict. A positive effect was found on change in teacher-reported parent involvement in school (d[subscript w] = 0.40), however, not on change in teacher-reported bonding with parents. Results suggest a potential preventive impact of the IY-TCM programme on change in teacher-student relationships and teacher-parent involvement when implemented as a universal preventive intervention in a regular school setting.