Background <p>School-based physical activity programs are widely promoted to address adolescent physical inactivity, yet their population-level impact is often limited by implementation challenges in real-world school settings. While implementation fidelity is frequently assessed, less is known about how teachers actively balance adherence and local adaptation while maintaining core program principles. This study examined the implementation of the FIT FIRST Teen (FFTeen) program in Danish lower secondary schools, with a particular focus on how conceptual fidelity was enacted through teacher-led adaptations.</p> Methods <p>An implementation-focused mixed methods study was conducted in six intervention schools participating in a cluster randomized trial. Implementation fidelity was assessed through systematic observations of FFTeen sessions at three time points using a structured fidelity framework. Semi-structured interviews with teachers explored adoption, implementation experiences, adaptations, and perceived outcomes, guided by Rogers’ Diffusion of Innovation theory. Data were analyzed using a deductive, framework-based approach integrating observational and interview data.</p> Results <p>FFTeen was implemented with moderate to high fidelity across schools, although procedural adherence varied substantially. Teachers frequently adapted session structure, activities, and organization in response to contextual constraints such as facilities, class composition, and scheduling. These adaptations generally preserved the program’s core principles of high intensity, inclusion, and enjoyment and were often associated with high student engagement. Teachers perceived FFTeen as offering clear pedagogical advantages over existing practices, particularly for engaging less-active students. However, sustained delivery of three weekly sessions was constrained by rigid timetables, competing curricular demands, and leadership support.</p> Conclusions <p>This study demonstrates how conceptual fidelity in school-based physical activity programs can be maintained through teacher-led adaptation rather than strict procedural adherence. Findings highlight the importance of flexible program design, professional judgement, and organizational alignment when implementing scalable physical activity interventions in schools. Rather than viewing adaptation as a threat to fidelity, implementation strategies should support teachers in enacting core principles within diverse and resource-constrained school contexts.</p>