Our society is at a pivotal moment where human impacts on the planet are undeniable, presenting urgent sustainability challenges like climate change, biodiversity loss, and security concerns (Schröder et al., 2020). Amidst these crises, rethinking educational models is vital for nurturing the civic engagement of our youth, who are the inheritors of these global issues (Wals, 2015) and the architects of tomorrow (Hickman & Riemer, 2016). In this context, the need for their empowerment is emphasised so that they are trained to transform unsustainable environmental, social and economic structures (Schank & Rieckmann, 2019). It is recognised that much is expected of young people when it comes to taking responsibility for creating a sustainable future (UN, 2015; UNESCO, 2014).

The transition toward a sustainable future requires fundamental changes in existing ways of thinking and acting. The most important tool for such a transition is education, especially in the context of achieving the Sustainable Development Goals (UN, 2015). There is a need to create appropriate mechanisms and conditions for educating new generations that promote the development of sustainability citizenship characteristics. These characteristics involve the interrelation between 1) cognitive, 2) socio-emotional, and 3) behavioral domains. Education for sustainable development (ESD) should motivate educational institutions to provide platforms where students can practice what they have learned, as captured by the phrase "walk the talk" (Holst, 2023). The aim is to equip young people with skills that enable them to actively participate in sustainable transformation (Rieckmann, 2021), and ultimately to cultivate sustainability citizens (UN, 2015).

Volunteering is recognized as a key factor in solving social issues, promoting solidarity and empathy, and redefining the fundamental structures of society (European Volunteer Centre, 2019; UNV, 2021). In this context, school volunteer programmes (SVPs) are recognised as platforms with the potential to cultivate sustainability citizens. SVPs are seen as models of ESD as they embody its main characteristics. The fundamental characteristics of ESD include holism and pluralism (Rudsberg & Öhman, 2010). Moreover, it is crucial for ESD models to incorporate experiential and transformative learning that promotes a shift in thinking, creative problem-solving, and the cultivation of reflective citizens (Rončević & Rafajac, 2012). Additionally, teaching in such programmes should be student-centered, connect course content, and promote interdisciplinarity and transdisciplinarity. In this sense, the connection with the community is also crucial (Wals, 2011). All these characteristics are necessary for the development of sustainability citizenship characteristics in students, particularly because previous research indicates that ESD programmes focusing only on certain elements have an impact on knowledge but limited impact on attitudes and behaviours (e.g., Boeve-de Pauw & van Petegem, 2017; Krnel & Naglič, 2009; Pirrie et al., 2006).

There is limited research on SVPs, particularly those exploring their relationship with the concept of sustainability citizenship among students. Given the increasing implementation of these programmes in Croatian primary and high schools, there is a need for comprehensive research to capture the complexity of this phenomenon. The research question is therefore: What experiences, processes, and activities that are part of SVPs, are encouraging high school students' potential in developing characteristic of sustainability citizenship and how?

This paper presents part of the results from a larger study conducted for the purpose of a doctoral thesis. It focuses on presenting the development of students' knowledge, attitudes, values, and behaviour in the context of school volunteer programmes.

A qualitative case study was carried out with the aim of comprehending the phenomenon of school volunteering programs (SVPs) and their significance in cultivating sustainability citizenship characteristics in high school students. To ensure the selection of information-rich case studies, a number of criteria were set: 1) integration of SVP into the school curriculum, 2) a minimum programme duration of one school year, 3) collaboration with external partners, 4) the linking of volunteer activities with the sustainable development dimensions, 5) accessibility of the programme to all students. The case study was selected as the research strategy due to its characteristics of comprehensiveness and a holistic approach. This approach is particularly suitable for analysing the phenomenon of SVP, which is still poorly researched. The subject of the case study, which answers the question of which case of the selected phenomenon to investigate, refers in this research to the SVP. The object or analytical framework of the research refers to the case of developing characteristics of sustainability citizenship. For the purpose of this research, six case studies were selected that also meet the conditions for analytical generalisation. A triangulation of data collection methods was used in this research. Focus groups were conducted with student volunteers, a total of 34 female students and 4 male students, between the ages of 15 and 18. Additionally, interviews were conducted with 7 SVP coordinators. Finally, a qualitative content analysis of school curricula was conducted to gain a comprehensive and deeper understanding of the institutional context in which SVPs take place. Thematic analysis was chosen for the data analysis. The analyses were conducted separately for each method through and across the cases. Based on the constructivist paradigm, the analysis sought to unravel latent content aligned with the established research strategy, aim, and research question. Thematic analysis, noted for its ability to provide a rich and detailed description of data, was considered particularly suitable for under-researched areas such as the field of this research. The analysis was conducted following a rigorous and systematic process in six phases: 1) familiarisation with the data, 2) generation of initial codes, 3) search for themes, 4) review of themes, 5) definition and naming of themes, 6) producing the report (Braun & Clarke, 2006).

The research findings clearly indicate that participation in the activities of the school volunteer programmes (SVP) encourages the maturation and personal development of the students. Student volunteers perceive themselves as more mature, evolved, and responsible. The SVP coordinators confirm this result and add that they observe a more pronounced maturity and resilience in the students who have participated in the volunteer activities. Coordinators work long-term and closely with most of the students at all stages of volunteering, which allows them to observe and recognise these changes in detail. In addition, students and, coordinators state that they feel fulfilled, useful, happy, and satisfied as a result of their involvement in school volunteer programmes. In relation to the characteristics of sustainability citizenship, the study reveals that participants in the SVPs had the opportunity to gain understanding of sustainable development and their role in promoting it. Within the socio-emotional domain, students developed skills, empathy, altruism, and solidarity. They developed a sense of collective identity, both within the SVP and a sense of belonging to a wider community. The results also highlight that students can perceive themselves and their opportunities in relation to others around them. Within the behavioural domain, the results shows that active participation in SVP led to pro-ecological, frugal, altruistic, and just behavioural changes in students. Particularly noteworthy is the result that all students, without exception, expressed their intention to continue volunteering after completing their school education. Within the SVP, students acquire the knowledge and values necessary for personal and social transformation in the direction of sustainable development. In such a pedagogical framework, they become capable of acting in favour of the common good. By learning about sustainable development, its goals and dimensions and developing various skills, students are empowered to deal with the uncertainty and challenges of sustainability.

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