In an ever-evolving society and consequently school context, teachers are increasingly challenged. It is important for a teacher to continue professional development to meet the needs of his students (Desimone, 2009). To achieve teacher professional development, the importance of long-term professional development initiatives is demonstrated (Merchie et al., 2018).  However, a sustained and ongoing implementation of long-term professional development initiatives in the school context are often uncertain (Stoll et al., 2006). In the context of lifelong learning, it is, however, essential to examine how active professional development initiatives can autonomously persist to ensure their sustainability. In this study, sustainability of professional development initiatives is conceptualised as their capacity to endure and remain effective over the long term (van der Klink, 2023). This study aims to investigate the conditions that can contribute to the sustainable continuity of Teacher Design Teams (TDTs). A TDT is described by Handelzalts (2009) as ‘a group of at least two teachers, from the same or related subjects, working together regularly, with the goal to (re)design and enact (a part of) their common curriculum’ (p. 7). More specifically, a TDT can be seen as a type of Professional Learning Community in which teachers engage in professional development by collaboratively designing curriculum materials for active use in the classroom (Binkhorst et al., 2015) Different studies (e.g., Binkhorst et al., 2015; Voogt et al., 2016) demonstrate which conditions have a specific impact on the professional development of teachers and the designed curriculum materials. However, it is also crucial to ensure the continuation of this professional development and explore conditions that can contribute to the sustainability of the TDTs.

This research is conducted in the context of the Project Integrated General Subjects (PGS), a course in vocational secondary education in Flanders (Belgium). This course integrates general subjects in a meaningful and project-based manner, aligning with the real-world and professional experiences of vocational students. PGS provides an interesting context for this study as it faces significant teacher turnover (Sierens et al., 2017). The sustainability of TDTs becomes an even greater challenge in this setting, given the necessity for the TDT to continue and be effective despite changes in teaching staff.

Over a period of two school years, a TDT programme was implemented and facilitated by the researcher across four secondary schools. The TDT programme is based on a preliminary study (Gryson et al., forthcoming) and comprised monthly sessions within the four school-based TDTs, where curriculum materials were designed, along with four sessions per school year in an overarching and supportive networked TDT. In the school-based TDTs, a participating teacher was trained as an internal coach to ensure the autonomous progress of these TDTs. The networked TDT, coached by the researcher, was primarily organised to facilitate mutual support among the different school-based TDTs, exchange of knowledge and experiences, and conduct peer supervision sessions for the coach-teachers. Additionally, a digital platform was established where teachers across the TDTs could share their progress and curriculum materials. At the initiation of this first phase of the research in the school year 2020-2021, a total of 14 teachers participated across the four school-based TDTs. In the third school year (school year 2022-2023), the second phase of the research, the two school-based TDTs that decided to continue autonomously with the TDT were investigated but no longer supported by the researcher. Both school-based TDTs were expanded with additional teachers, which resulted in a total of 12 participating teachers across the two TDTs. To collect data, a semi-structured interview was conducted with the participating teachers each school year. Additionally, reports from the school-based TDTs were collected, and verbal reports from the coach-teachers were transcribed. Since the researcher was present during the networked TDT meetings and some school TDT meetings, observation reports and notes from informal conversations were also included in the data collection. Throughout the entire research period, informed consent was obtained from each participant. The data were analysed using reflexive thematic analysis (Braun & Clarke, 2019). In reflexive thematic analysis, the researcher's subjectivity is regarded as a source and not immediately as something negative. Given the researcher's close involvement during the TDTs, the application of Braun and Clarke's (2019) reflexive thematic analysis acknowledges its guidance in developing the results. The findings were constructed through an intensive, iterative, and theme-based analysis.

This study demonstrates that various conditions have an impact on the sustainability of TDTs. It is demonstrated that both motivated teachers and the need for a core team are important, along with a clear focus during regular TDT meetings. However, it strongly emphasised that a crucial role is assigned to the school leader and the coach. Both stakeholders play a vital role in supporting TDTs and ensuring that the work of participating teachers is perceived as valued and meaningful. For the school leader, this support should manifest through allocating time and resources for teachers, as well as expressing confidence and appreciation. The support the coach needs to provide is mainly related to a proactive approach in organising and guiding the TDTs, with a pronounced need for an internal coach. Subsequently, the results also indicate that for all participating teachers in a TDT, the research-based foundation of TDTs must be clear. This leads to teachers perceiving their work as meaningfully anchored. Additionally, it is emphasised that for the sustainability of TDTs, not only the autonomous progress of one's own school-based TDT is essential but also cross-school collaboration remains important. The research also indicates that the context in which the TDTs are initially implemented and subsequently routinised is important. The influence of the Covid-19 measures and ongoing national educational reform at that time greatly impacted the frequency and way teachers could meet in the TDT, as well as the choices and iterative adjustments made to the curriculum materials.

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