Polarisation between the least and most active pupils is increasing both in Denmark and worldwide, and the least active pupils often have poorer mental health and low physical self-esteem. However, the least active pupils' perspectives on physical activity in school are underrepresented in the research. This study explores how the least active pupils describe the purpose of physical activity at school and how active they consider themselves to be in relation to their classmates. To establish the above, we draw on Jenkins' theory of social identity, which points to the dialectics between internal identification and external categorisation. Three schools were selected for a qualitative multiple-case study, focusing on the least active Year 3 pupils (aged 9-10). Thirty individual semi-structured interviews with these pupils were conducted, combined with 15 days of participant observations, at each school in 2018. Data were analysed using an abductive approach. The findings showed that the least active pupils' descriptions of why they were active mainly referred to narrow understandings of physical health in line with broader societal explanations. Further, the pupils depicted a stigmatic categorisation of overweight and inactive pupils and described them as a group they did not belong to. Instead, they identified themselves with the active pupils, for example, in stating that they were almost as active as the 'football boys'. To change pupils' narrow identifications and stigmatic categorisations, school professionals should focus less on health as an argument for physical activity.