Purpose: Proper handwashing helps prevent the spread of communicable diseases. The aim of our study was to analyse and compare children's knowledge and skills in hand hygiene before and after school interventions in order to evaluate the effectiveness of our peer education programme. Materials/ methods: In our longitudinal study, short- and long-term changes in the knowledge, hand-washing skills and health behaviour of 224 lower, upper and secondary school students were assessed. Our measurements were performed with a self-administered, anonymous questionnaire and the Semmelweis Scanner. Results: As a result of the intervention, the proportion of correct answers increased significantly both in the short term and in the long run compared to the input measurements, but age differences did not disappear for most variables. There is a difference in the process of learning theoretical knowledge and practical skills. Areas not used for handwashing in the paediatric population are different from those described for adults in the literature. There was no significant difference between the mean scores of the right and left hands. Conclusions: There was a significant positive change in both theory and practice of handwashing. In education, emphasis should also be put on long-term retention of theoretical knowledge in age-specific health promotion programmes within the paediatric population.  Kulcsszavak: hand-hygiene, handwashing skills, handwashing knowledge, peer education, school intervention