Background <p>Many implementation determinants of school nutritional interventions have been identified, but a deeper understanding of their interplay within a system is needed. The present study aimed to investigate how the Norwegian School Fruit Programme, particularly the No Parental Payment Programme (NPPP), is being implemented in schools and how it interacts with the school system structure and dynamics as perceived by school personnel and pupils. Moreover, it aimed to understand the dynamic interplay between the determinants of the Consolidated Framework for Implementation Research (CFIR) in the NPPP’s context.</p> Method <p>The overarching methodology was qualitative system dynamics modelling, with data collected through a series of group model building (GMB) workshops at four schools offering the NPPP. Using GMB scripts, facilitated discussions between school personnel and pupils were held for a better understanding of the NPPP’s implementation dynamics and to create diagrams illustrating how implementation determinants interact with each other and each school’s unique context. The individual school diagrams were compiled into a single diagram. The CFIR was subsequently mapped on to this diagram.</p> Results <p>Lack of communication with pupils, the food stores outside schools, and the NPPP’s design weakened the reinforcing mechanism that fostered pupils’ eating of fruit. Not eating enough of the fruit increased implementation costs because schools had to deal with leftover fruit and littering issues. Implementation costs caused schools to adopt implementation processes, like reflecting, engaging, and adapting to reduce the costs, although some of these did not work or counterintuitively decreased pupils’ eating of fruit. The success of these processes was contested by the design of the NPPP and mechanisms that were shaped by the goals and priorities of the schools.</p> Conclusions <p>This research employed qualitative system dynamics to investigate NPPP implementation dynamics. Recommendations for improving implementation include aligning NPPP’s design with pupils’ preferences, monitoring eating of fruit, and involving internal stakeholders (pupils and school personnel) for gathering feedback, and external stakeholders (municipalities) for monitoring.</p>