The purpose of the present study was to investigate the level of physical activity (PA) in an all-boys school and, using a mixed method design, to identify factors that facilitate or hinder PA engagement. Participants (n = 382) completed the Three Day Physical Activity Recall (3DPAR) questionnaire to enable examination of the pattern of their PA and the amount of time spent on sedentary activities. To understand further the motivation behind their level of PA, 24 participants with the highest and lowest 3DPAR scores were selected for focus groups. Quantitative data of the PA levels of the all-boys sample of this study and those of a mixed sample from a study by Lee and Trost were analysed using percentage means with 95% confidence intervals, with a 2x2 analysis of variance being used for the comparisons. The focus group data were transcribed and thematic analysis was performed on the transcripts. The quantitative results revealed evidence that male students in the current sample were more active in comparison with the students from the mixed sample. The qualitative results indicated that PA participation was facilitated by factors such as intention to maintain health and fitness levels, a strong school sports culture, regular exercise with friends and family, and an individual's exercise history. In contrast, academic workload, poor exercise history and after-school non-PA related activities were identified as factors that hindered PA participation. The findings are discussed in relation to wider school decisions and policies that have an effect on PA among adolescent students.