Background: Physical Activity 4 Everyone (PA4E1) is a whole-school physical activity program, with demonstrated efficacy (2012–2014). PA4E1 was adapted (scaled-up) and tested in a scale-up trial (2017–2020). This process evaluation study of the scale-up trial had 2 aims. First, to describe the acceptability, appropriateness, and feasibility of PA4E1 in the scale-up trial, from the perspective of school staff involved in the program management and delivery. Second, to generate themes that may explain school staff assessments of acceptability, appropriateness, and feasibility. Methods: Data were collected at various time points throughout the 2-year implementation phase. Online surveys were collected from In-School Champions, Head Physical Education teachers, Principals, and Physical Education teachers (quantitative data). Focus groups and interviews were conducted with In-School Champions, Principals, and Physical Education teachers (qualitative data). Existing published data on website engagement, adaptations, modifications, and the scale-up trial primary outcome (implementation of physical activity practices) were triangulated with the quantitative and qualitative during analysis, to generate themes. Results: School staff delivering PA4E1 reported it was highly acceptable, appropriate, and feasible. Seven themes were generated relating to acceptability, appropriateness, and feasibility. The themes related to how the program was funded, the delivery modes of implementation support, the identification of easy-wins, the recruitment of the right in-school champion, facilitating principal buy-in, mitigating the impact of school staff turnover, and engaging the whole school. Conclusions: Recommendations are made to inform future adaptations for PA4E1 and potentially school-based physical activity programs more generally. The findings may inform future scalability assessments of the suitability of programs for scale-up.