This study used a quasi-experimental design to evaluate the efficacy of the universal, school-based VIP-Makkerskap [VIP Partnership] programme. 1101 students in a test group and 734 students in a control group completed questionnaires one week (t1), ten weeks (t2), and six months (t3) after programme implementation. A one-way ANCOVA showed that at t2 and t3, students in the test group reported significantly higher social classroom environment scores than the control group, but the effect sizes were small (d = .10 and .09, respectively). Further analyses showed that five of the ten test schools accounted for the increase in the outcome variable from t1-t2. In these schools, a greater proportion of teachers had used the programme since its beginning in 2015, compared to the schools that did not report an increase. The results suggest that teachers' experience in using VIP Partnership is a key component of the programme's effectiveness.