In addition to Multi-Tiered System of Supports (MTSS) and Universal Design for Learning (UDL), one of the transformative strategies used in various inclusive school reforms to improve outcomes for all students has been shared teaching by general and special education teachers (Sailor, 2015). This shared teaching has been referred to as co-teaching. Co-teaching has been defined in different ways depending on the focus on the professionals involved, the learners served, or the shared tasks (Cook & Friend 1995; Villa et al., 2004). Fluijt et al. (2016, p. 197) define it as 'multiple professionals working together in a co-teaching team based on a shared vision, in a structured way and over a long period of time, with equal responsibility for the teaching and learning of all students in the class'. This concept understands co-teaching for all learners and does not limit it to the joint practices of a generalist and a special education teacher.

Friend et al. (2010) proposed different models that co-teaching takes within classrooms. The degree of reciprocity, equality, roles and co-responsibility of functions varies according to the models, with team teaching being the most egalitarian. The models used should vary according to the purpose, as some are more appropriate than others (Beyers et al., 2013). It is worth noting that when there are two teachers in the classroom, the most common model is one teaching and one assisting, as is the case with non-teaching support staff (Jardí et al., 2021).

There is no doubt that teamwork and quality interactions between the different educational actors in the classroom are fundamental to the flow of shared learning (Jardí, et al., 2022). Inclusive education requires openness and sharing proposals with the community based on authentic and spontaneous collaborative cultures that go beyond contrived interactions enhanced by administrative power (Hargreaves & Dawe, 1990). Ya en los noventa, Little (1990) discussed the dominance of individualism among teachers and developed a continuum of collaboration based on the interdependence of teachers, scaled from simply storytelling or sharing ideas to the authentic joint work consistent with the definition of teamwork required for co-teaching.

In fact, collaboration during lesson planning was a significant predictor of student achievement in the United States (Reeves et al., 2017). For this reason, it is no wonder that the literature on co-teaching emphasised the relevance of co-planning, co-construction, co-evaluation, co-respect, co-responsibility (Pratt et al., 2014; Scantlebury et al., 2008).

Consensus has been reached on the importance of understanding that achieving favourable partnerships is a process that involves time and effort (Jurkowski &  Müller, 2018; Pratt, 2014; Rytivaara et al., 2019). To this end, Pratt (2014) shed light on the process that co-teachers can follow to build interpersonal relationships. She proposed a model called Achieving Symbiosis, composed by three main phases: initiation, symbiosis spin and fulfilment. Both the second and third phases of the Achieving Symbiosis model are conceived as a process that never ends, as partners should always work on their partnership. Along similar lines, Rytivaara et al., (2019) proposed the following three main steps: 1) the commitment to building a partnership with a colleague; 2) learning to share their practical knowledge through discussing their personalities, professional backgrounds and classroom practices; and 3) a joint reflection. However, more research is needed to know how all these interpersonal and external factors fit together to ensure effective teamwork, especially in educational systems where co-teaching is in its infancy or under-researched. To this end, the aim of this communication is to explore the main interpersonal factors that influence co-teaching relationships in the cases studied and, consequently, inclusive educational practices.

This phenomenological study is part of a wider qualitative research project that aims to explore different experiences of co-teaching and the perceptions of the teachers who implement them. Semi-structured interviews were conducted with 81 teachers (79% female) from pre-school (n = 3), primary (n = 20), secondary (n = 50), high school (n = 3) and vocational education (n = 8) from 21 schools in Andalusia, Cantabria, Catalonia, Extremadura, Madrid, the Basque Country and the Valencian Community (Spain). The sample was purposive, with the criterion of obtaining diverse experiences of the implementation of co-teaching. The interview script, consisting of 14 topics, was subjected to a group review. The research focused on identifying how co-teaching practices emerged in the different schools or teaching teams and how it was implemented, taking into account all its phases (planning, teaching, evaluation, reflection and professional development). We also inquired about contextual and sustainability factors, the perceived impact on their practices, on learning and on the care of all students, with a particular interest in the role of the special education teacher. The theme studied for this paper focuses on the interaction between professionals. After a first reading, the researchers divided the themes and proposed a system of categories and subcategories, which were discussed by the research team until a consensus on definitions was reached. The process of analysis was therefore deductive and, in the case of this paper, without the addition of emerging categories. To ensure the reliability of the coding system, several rounds of peer consultation were conducted until consensus was reached (Brantlinger et al., 2005). Finally, the degree of agreement of the resulting analysis was calculated using Cohen's kappa index by comparing it to that of a researcher independent of the process, accepting K greater than 0.8. Quotations from discrepant subcategories were discussed until consensus was reached. The resulting categories were: (1) relational descriptors, (2) conditioning factors, and (3) ‘soft skills’. The subcategories of 'relational descriptors' are 'open communication', 'professional compatibility', 'affinity', 'trust', 'equality' and 'respect and appreciation', 'sense of belonging', 'conflict'. The conditioning factors were categorised according to whether they were ‘facilitators’ or ‘stressors’. Finally, the subcategories of ‘soft skills’ are ‘adaptability’, ‘openness to mutual learning’, ‘assertiveness’, ‘effective communication’, ‘collaboration’, ‘creativity’, ‘empathy’, ‘constructive feedback’, ‘conflict management’ and ‘motivation’.

Our findings show that achieving effective partnerships is a complex process that requires time, effort and external support as A1004 expressed: It is indeed necessary to come to an agreement. There are teachers with whom it is easier to come to an agreement and there are teachers with whom it is not so easy. I am lucky that with the teacher I have to agree with, the one in the other classroom, we have worked together for many years and we have a phenomenal relationship and we agree very well. But it's true that there are times when we have our tug-of-war with other teachers. So in the end we have to find common ground. Personal affinity, open communication, a sense of belonging and professional compatibility are key to feeling comfortable in the classroom. However, some of these relational factors are perceived in different ways, so co-teachers should clarify expectations and work explicitly on adapting to each other's styles and preferences. Nevertheless, there are a number of soft skills that all teachers, but especially those who co-teach, should work on. Teamwork and communication skills, adaptability and empathy were the most commonly reported. The facilitators analysed tells us that co-teaching is based on flexibility, communication and coordination between teachers, promoting collaboration and joint planning to strengthen teaching. Good relationships between teachers, a supportive school culture and administrative support are fundamental to the success of this enriching educational practice. Some of the challenges reported by participants include lack of allocated timetables, lack of resources, resistance to change and coordination difficulties. These barriers need to be overcome through training, institutional recognition to promote effective collaboration between teachers for the benefit of learners, and the systematisation of procedures to optimise co-teaching from planning to evaluation.

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