Education for sustainable development aims at strengthening levels of knowledge, attitudes, and behaviours beneficial for sustainable development which are crucial to tackle urgent sustainability-related global challenges. In this study, we present the results of a one-year panel study measuring levels of sustainable development knowledge, attitudes, and behaviours in primary and secondary schools in Flanders (Belgium). During this year, a large-scale project on sustainable development was implemented in these schools. We test whether this intervention increased pupils' knowledge, attitudes, and behaviours on sustainable development. Furthermore, we look for heterogeneous effects depending on pupils' gender and age. The results show that there is indeed an increase in levels of sustainable knowledge, attitudes and behaviours after the ESD intervention, and that there are significant differences between boys and girls on the one hand, and primary and secondary school pupils on the other. However, we find no evidence that the intervention mitigated these differences.