Teachers' professional learning is a desired outcome. Contemporary approaches view it as heavily affected by implicit factors (Evans, 2019). Namely, learning about teaching by practicing it in a specific context influences what teachers learn and how they develop as professionals (Opfer & Pedder, 2011; Webster-Wright, 2009). Mentoring student-teachers is a practice that has a potential of making mentor-teachers explicate such implicit factors. It provides mentors with learning opportunities (Smith & Nadelson, 2016), especially when they build collaborative relations with their student-teachers and together reflect upon their practice (Feiman-Nemser, 2001; Norman & Feiman-Nemser, 2005).

Co-teaching can serve as a means for mentoring student-teachers and for creating learning opportunities for mentors'. In co-teaching mentor-teachers work alongside student-teachers and teach together. In many cases cooperation between mentors and student-teachers is limited, but when a full-fledged collaboration evolves, partners plan, practice, and reflect together (Guise, Habib, Thiessen, & Robbins, 2017). Such collaboration promotes the articulation of thoughts and supports learning opportunities (Nilsson & van Driel, 2010; Van Velzen, Volman, Brekelmans, & White, 2012).

In short, we understand that mentoring student-teachers and co-teaching alongside them provides mentors with learning opportunities. However, there is still a gap of knowledge in regard to the role that student-teachers play in creating and shaping these learning opportunities.

Influencing others' professional learning is an aspect of professional agency. Generally speaking, professional agency includes two main aspects. One aspect is the capacity to perform professionally according to one's knowledge, skills, beliefs and identity (Eteläpelto, Vähäsantanen, Hökkä, & Paloniemi, 2013; Turnbull, 2005). The other aspect is the capacity to influence professional practice through influencing processes of professional learning (Pyhältö, Pietarinen, & Soini, 2012; Soini, Pietarinen, & Pyhältö, 2016). In other words, to be agentic in that sense means that a teacher can actively create learning opportunities and shape them (as well as their outcomes) for either himself or his colleagues.

Some prior studies discussed student-teachers' enactment of professional agency (Price & Valli, 2005; Pyhältö, Pietarinen, & Soini, 2015; Ruohotie-Lyhty & Moate, 2016). These studies focused on student-teachers' capacity to perform as professionals in classrooms or influence their own professional learning. However, such studies do not pay much attention to the impact student-teachers have on the professional learning of their mentors or other senior teachers. A few studies that did look at student-teachers' (and novice teachers') influence upon other teachers' professional learning found that student-teachers felt less agentic in that sense (MacPhail & Tannehill, 2012; Toom, Pietarinen, Soini, & Pyhältö, 2017). This might imply they can serve only a passive role within the processes of professional learning experienced by their mentors. But it would be absurd to assert that mentoring leads to professional learning opportunities based only on the practice of the mentor, while the mentees' actions bear no effect upon it. We suggest that a more elaborate understanding of student-teachers' enactment of professional agency would allow us to unpack the unique role they play in influencing mentors' learning.

To gain such an elaborate understanding we focused on three dyads of mentor-co-teachers and student-teachers, and asked two research questions:

• Did student-teachers provide mentors with professional learning opportunities?
• To what extent did student-teachers enact professional agency in shaping their mentors’ learning opportunities?

The research is based on a multiple case study approach (Yin, 2014), and comprises three cases, each focuses on one mentoring relationship from diverse perspectives. This approach allowed us to compare cases (Bartlett & Vavrus, 2017) in order to develop a model that explains how student-teachers acted as professional agents. Context The mentoring relationships were between mentor-teachers and student-teachers. They participated in a university preparation program that operates under an experimental teacher preparation policy framework called Academy-Classroom initiated by the Ministry of Education in Israel. The framework requires that student-teachers would go an intensive practicum consisting of two days per week during the school year with a mentor-teacher who co-teaches alongside them. Participants The case study methodology requires careful sampling (Creswell, 2013). Thus, purposeful sampling was applied (Patton, 2002) based on university supervisors and school vice-principals' recommendations for accomplished mentors. Such recommendations-based sampling is common and was used in similar studies (authors, 2014; Williams, 2014). Participants in each case included: the mentor-teacher, the student-teacher, the university supervisor, the preparation program coordinator, the school vice-principal, a teacher-colleague from school. Data collection In each case, we interviewed each of the participants and conducted an observation of the interaction between mentors and student-teachers in schools. In total, we gathered approximately 21 hours of interviews and 12.5 hours of observations. Using multiple data points of diverse participants as well as our own observation allowed triangulation of the findings (Flick, 2018) and uncovering of implicit learning opportunities. Analysis We analyzed in two stages. We first looked for indications for professional learning opportunities. These included mentors' willingness to consider using new practices or knowledge and stimuli that act upon mentors' practice or knowledge having a potential of influencing them. However, we interpreted such indications as learning opportunities only when two different data sources pointed at the same practice or knowledge. After identifying mentors' learning opportunities, we looked for factors that may better explain student-teachers' capacity to create and shape these learning opportunities. We focused on relationships between mentor-teachers and student-teachers, student-teachers' actions, and resources that student-teachers used and allowed them to influence mentors' learning.

Mentoring dyads provided mentors with opportunities for professional learning. Student-teachers were able to create and shape these learning opportunities. Therefore, we infer that student-teachers acted as professional agents. Each student-teacher influenced his mentors' learning in a different way. One embraced the role of an entrepreneur. His vision of good teaching and the preparation program's assignment requiring him to create a pedagogical initiative both served as resources. He utilized them and initiated an extracurricular course that he co-taught with his mentor, which allowed the latter to experience teaching methods that differ from those she was used to. Another student-teacher acted as a broker. The pedagogical knowledge she learned at the university served as a resource, which she introduced to her mentor who was curious to examine and learn from. The third student-teacher capitalized on her in-depth knowledge of the subject-matter in English as a foreign language lessons, being the only native speaker in the classroom. She acted as a content expert and contributed to the mentor's content knowledge who was glad to receive her inputs. Synthesizing these three cases into a model may help explain the enactment of professional agency by student-teachers. The process starts with the utilization of resources available at the student-teacher's disposal. This in turn allows him to enact professional agency and create and shape learning opportunities for his mentor.

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