Students appreciate group work as a means of learning and several studies also suggest that students who work together in groups have better learning outcomes. Nevertheless, teachers still seem reluctant to use group work as a pedagogical tool in the classroom. The main focus of this qualitative study is to address group work as a classroom activity from the teachers' perspectives, and more specifically to ascertain why teachers are reluctant to use group work as a mode of working in education. Data were collected by means of focus group interviews with teachers from three different schools, and the analysis was carried out using grounded theory. The uniqueness of this study is that the results show that teachers' presumptions, together with their mode of procedure and their negligible reflections regarding subject knowledge as a learning outcome when using group work, have a decisive influence on their willingness to use group work.