The purpose of this thesis was to describe and understand the phenomenon of school volunteer programmes, and the research examined the experiences, processes, and events that take place in this programme that promote the development of sustainability citizenship characteristics in high school students. In this research, a case study was conducted that included six school volunteer programmes. Semi-structured interviews, focus groups, and a content analysis were used as data collection methods, and a thematic analysis was used to analyse the data. Key mechanisms for promoting the development of sustainability citizenship among students were identified. A positive school context characterized by cohesion and support from principals and teachers is identified as a foundational framework. Central to this development is the frequency of performance and a rich array of volunteer actions that match students' interests. Linking volunteer actions to cross-curricular themes deepen their meaning, while partnership with external collaborators ensures the authenticity of the topics covered. Programmes have proven resilient in the face of various, often unpredictable, circumstances. Such programmes ensure that students become resilient, empowered, and ready for action regardless of obstacles. Throughout the process, equal relationships between students and coordinators become critical in planning volunteer activities. This not only prepares students for active participation in civic activities, but also provides them with the opportunity to engage in the role of a sustainability citizen in practice. Finally, coordinators play a central role in this process. In their role as mentors and role models, they provide invaluable support and guidance to student volunteers, shaping future generations of sustainability citizens. In addition, this thesis provides recommendations for improving these programmes in secondary schools in terms of developing the characteristics of sustainability citizenship.