Study protocol for a quasi-experimental evaluation of the whole school approach FREI DAY

Background The Sustainable Development Goals combine health, social justice, sustainability, and environmental protection. Schools offer a suitable setting for linking these goals and incorporating them into the development of children and youth. The FREI DAY is a whole-school approach in German-speaking countries and at German schools abroad, designed to improve lifestyles and living conditions in both health and environmental sustainability, and to build schools’ capacity to transform into healthy and sustainable organisations. The FREI DAY is theory-based and aligned with the United Nations Sustainable Development Goals. This study examines the effectiveness of FREI DAY and the conditions necessary for its success, providing recommendations for its implementation and further development. Methods The study is designed as a natural experiment with a sequential design. Over a period of three years, at least 60 secondary schools, including vocational schools, and 30 primary schools will be surveyed during three survey periods. The schools themselves decide whether and when to implement the eight-phase FREI DAY intervention. This means that schools can switch from the control condition to the intervention condition and vice versa during the study period. In each survey year, at least 20 students per school will be surveyed, i.e., around 1,200 students per wave. In addition, five school key informants per school will be surveyed annually, making 300 per wave (open cohort design). At intervention schools, at least three members of the school board per wave will also be surveyed. Changes in the results will be evaluated using hierarchical linear models. Discussion This is the first study to evaluate the effectiveness of FREI DAY. We expect FREI DAY to improve students' well-being, life skills, environmental awareness and environmental responsibility. In addition, it is expected to strengthen the transformative capacity of schools, create a new culture of learning and sustainability, and strengthen collective resilience. Trial registration: This study was registered in the German Clinical Trials Register: DRKS00036310 on August 07, 2025.