Angela de Jong

50 knowledge, and ideas are exchanged, resulting in mutual development (Owen et al., 2008; Torfing, 2019). Vertical processes pertain to collaboration that cuts across different organizational levels, functions, and hierarchies (Torfing, 2016), which in schools would be between teachers and school principals. Horizontal processes imply collaboration between persons and organizations at the same level, which in schools would be between teachers. Collaborative innovation is argued to strengthen and improve all different phases of an innovation process, namely the phases of problem definition, idea generation, idea selection, implementation, and diffusion (Sørensen & Torfing, 2016; Torfing & Triantafillou, 2016; Van de Ven et al., 1999). However, collaboration is not easily fostered in the educational context, since schools are loosely coupled systems (Orton & Weick, 1990). It is common for teachers to mainly focus on their own classroom (practice) and tasks, resulting in their work activities being largely autonomous and isolated (e.g., Admiraal et al., 2012). Consequently, innovation in schools is often seen as an isolated activity of one teacher or a minority of teachers who decide to initiate change (Sales et al., 2016). This hinders innovation, since we know that teachers in schools with collaborative structures and cultures tend to learn more from each other as compared to schools without collaboration (Drago-Severson, 2012). 3.2.2. Leadership and leadership practices in collaborative innovation In both organizational and educational theories, leaders are thought to have a vital role in leading innovation processes (Bush & Glover, 2014; Fullan, 2016; Torfing, 2019). A recent review of twenty years of effective school leadership literature has demonstrated the importance of an active support of instruction and effective communication, as well as the positive influence of school principals’ leadership on building organizational climate and culture, trust, and collaboration (Daniëls et al., 2019). In the context of collaborative innovation, Torfing (2016) for instance theoretically studied leadership. He identified three types of leaders who can stimulate collaborative innovation in the public sector: Conveners (e.g., spur interaction), Facilitators (e.g., promoting collaboration), and Catalysts (e.g., prompting actors to think out of the box). Sørensen & Tor ng (2016) and Torfing (2019) acknowledge the guiding role of leaders in collaborative innovation and call for further research on leadership in collaborative innovation (e.g., Torfing, 2019). In order to gain more insights into leadership of school principals in collaborative innovation, we use the concept of leadership practices. Leadership practices of school

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