Education for Sustainable Development (ESD) is strongly encouraged by major global policy frameworks. However, we still know very little about how these macro-level guidelines are actually implemented on the ground in secondary schools within highly diverse, metropolitan areas. This empirical study uses a comprehensive analytical framework called "ESDUC" to closely observe how sustainability is integrated into Dutch-speaking secondary schools in Brussels, while seeking to understand the specific factors that either hinder or facilitate this process. The research is built upon a qualitative approach, using in-depth interviews and focus groups with key school actors, namely pupils, teachers, and school leaders.

The results show that the integration of sustainable development remains incomplete, uneven, and highly fragmented across the different schools. The main issue identified is that sustainability is almost always viewed through a narrow ecological and environmental lens—focusing mostly on practical actions like waste sorting or energy reduction. Conversely, more transformative, holistic approaches that adapt to the local culture, encourage community engagement, or address social equity remain very weakly developed.

Furthermore, the study demonstrates that the intense urban reality of Brussels heavily shapes this situation. Key challenges such as widespread multilingualism, the socio-economic vulnerability of many families, and a structural mismatch between the pupils' daily lived experiences and traditional school practices make implementing a uniform programme very difficult.

In conclusion, this research highlights that we cannot simply impose global sustainability agendas from the top down. It is absolutely essential to adapt these initiatives to the demographic, cultural, and social reality of the local context. Ultimately, it offers practical and accessible pathways to help schools in Brussels and other major metropolitan areas provide a sustainability education that is truly inclusive, just, and meaningful for all students.