The author wanted to test the effects of preventive mental health programmes in schools and established a longitudinal study with a test group and a control group, using Solomon's method. Data was collected through questionnaires prior to intervention and at 1, 6, 12, and 24 months after the intervention. The size of the effect on the various indices were estimated in terms of (a) differences in improvement of total percentage scores and (b) Cohen's d. From to to t1, t2 and t3 the intervention group showed significantly greater progress in six out of seven knowledge indexes, and 12 months later we found significant effects on the level of mental health problems.