Background: Adolescent girls are likely to be less active than adolescent boys. A range of factors including multiple psychosocial aspects are thought to influence their engagement in physical activity (PA). This study aimed to explore adolescent girls' perceptions and experiences of school-based PA to inform a subsequent intervention, the Girls Peer Activity (G-PACT) project. Methods: The Youth Activity Profile was used to assess adolescent girls' current PA levels. Open-ended questions were used to investigate girls' perceptions and experiences of school-based PA. Focus groups stratified by PA level were then conducted to explore their perceptions and experiences in depth. The focus group data were analyzed using inductive thematic analysis. Results: The master themes emanating from the focus groups were noncompetitive activities and after-school sport culture for lower active girls. Higher active girls' master themes were PA perceptions and physical education. Regardless of activity level, participants reported greater enjoyment from PA when participating with friends and having choice over activities provided within the school setting. Conclusions: The findings highlight the importance of choice, peer groupings, noncompetitive opportunities, and PA competence to adolescent girls' school-based PA behaviors. The school environment can support and restrict girls' engagement in PA. The findings will be applied to the design, content, and implementation of the G-PACT project.