A range of transition research suggests that students undergo a dip in performance subsequent to an educational transition. In this paper, through consideration of ethnographic data and recent situated theoretical conceptualisations of learning and teaching, we explore the mechanism of this phenomenon. We argue here for the use of situated learning models where all knowledge is understood as being contextualised in the activities and practices of both the learning situation and the wider societal frameworks which construct that situation. We explore children in transition and the concomitant learning and teaching practices in a UK school. We demonstrate that knowledge and performance is mediated by the practices that the new students must acquire in order to participate in their new community. The data follow a group of year 7 children who are new to the school and their teacher over a three week period engaged in work on averages. In their efforts to adopt the practices dictated by the teacher, the data demonstrate that conceptual and procedural understanding shifts from one of confidence to one of hesitancy and questioning. Understanding of children in transition, school performance and the attendant teaching practices are discussed in light of the data