School leaders experience numerous school policy and school development challenges under the pressure of a complex social context (Plavčan, 2020). Furthermore, they are pressured by their societal duty to provide quality education (Gurr & Drysdale, 2020). To best perform this challenging task, school leaders would benefit from the support and input from peers within a sounding panel as well as from concrete policy stimuli that encourage quality and innovation (Vekeman et al., 2022). Inter-school networks can also provide a significant added value (Brown & Poortman, 2018; Harris & Jones, 2021), as they increasingly encourage schools to engage in structural collaboration (Brown & Flood, 2020; Levin et al., 2020; Rincón-Gallardo & Fullan, 2016). In Flanders, school communities offer such an opportunity for collaboration.

Building quality partnerships between the school leaders of a school community is not self-evident (Azorín et al., 2020; Harris & Jones, 2021). Because of its multidimensional and multilevel character, it is difficult to define, develop, and operationalize a PLC (Antinluoma et al., 2021). How do we initiate and sustainably support them? What difference does a professional development program initiating and supporting professional learning communities (PLCs) for inter-school collaboration make in terms of depth of (peer) learning and longevity?

Research on methodologies to intensify collaboration within existing inter-school networks is limited (Chapman, 2013). We examined how PLCs as a mode of formal collective learning (Schelfhout, 2017) develop within existing school networks throughout a professional development trajectory (PDT). We explored the perceived (learning) outcomes and which variables influence longer-term sustainable development.

The PLCs originated both in the context of school communities (formally government-based) and of self-selected inter-school networks, allowing a comparison between two organizational forms of school networks.

The study used a mixed methods research design. The combination of quantitative and qualitative data increases relevance and provides an opportunity to substantiate the relationship between variables. Before the PDT, participants completed an initial questionnaire. Open and closed questions surveyed to what extent participants already participated in structural inter-school networks, and whether a need for more cooperation within their school community existed. After the first year of training, the desire for the continuation of a PLC after the end of a PT was examined by using scale questions. At the end of the two-year PDT, a survey with closed and open-ended questions was conducted, focusing on experiences with the organization and approach of the PLC, the perceived effects on the outcomes, and the desire for sustainable continuation. Furthermore, experiences with participation as an inter-school network were surveyed. 133 of the 138 participants (n=96%) who participated during the second year completed the final survey. In-depth interviews with school leaders were organized in May 2023, aiming to further question and explain trends that appeared from the quantitative data collection. The semi-structured online interviews were conducted using a question protocol based on the literature review and observations during the PLC meetings (Morris, 2015; Seidman, 2006). In-depth interviews were recorded with participants' consent. A total of 42 school leaders, five superintendents, and two participants combining school leadership with a position as a superintendent participated, evenly distributed across the different PLC groups.

Results indicate that the quality of collective learning increased significantly during the two-year trajectory. Most explanatory for further sustainability of the PLC as a professional network for school leaders is the organization and didactic approach of the PLC during the PDT and the approach used by the process coach. The initial situation and the facilitating role of the inter-school network as experienced by the participants influence structural choices regarding future continuation and approach. Linked to the initial situation and the facilitating role of the inter-school network, at the end of the PDT differences are observed in how the continuation of a PLC is ensured, more specifically in terms of the organization and approach as well as expected commitment and shared leadership. In school communities where the superintendent played a facilitating role before and (participated) during the PT, the PLC will be continued, although it is not always clear what exactly this facilitating role will entail and who should fulfill it. Regarding the choice of an (external) coach, the experience with one's process coaching during the PT, the expertise in the field of process coaching among PLC participants, the availability of coaches from the educational advisory service, and financial resources are taken into consideration. It is recommended that the participating school leaders and superintendents make conscious and well-founded choices for the sake of the quality of collaborative learning and invest in sustainable collective learning. Further longitudinal research into the sustainability of PLCs within inter-school networks and the quality of coaches is recommended.

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