Purpose: In 2014, the Danish Government introduced a new public school reform, which included implementation of 45 min of daily physical activity (PA) within the academic classroom curriculum. The purpose of the present study was to explore school staff's perceived barriers to implementation of a national PA policy. Method: A mixed-methods approach using a questionnaire and semistructured interviews was conducted. A total of 198 teachers and 26 school management team members (principals, deputy principals, and leading teachers) from 31 schools completed a questionnaire, and 11 school management team members were interviewed. The socioecological model was used as a theoretical framework to examine the results. Results: A total of 15 different barriers were identified and reflected within all levels of the socioecological model. "Facilities," "motivation," and "time" were the most prominent barriers identified. Conclusion: Development and deployment of a national PA policy needs to be done in cooperation with consumers from all levels within the socioecological model to ensure successful implementation.