A randomized controlled trial was conducted comparing the effects of a biopsychosocial course (PRESTO-Play) vs. physical activity promotion (PRESTO-Fit) to reduce disability related to musculoskeletal disorders in music students. The current study provides an external validation and a formative and process evaluation, allowing for a better interpretation of results. First, a group of experts was asked to complete a structured evaluation of design and content of the trial. Second, quantitative and qualitative data were analysed from different stakeholders (students, therapists and conservatory staff) using questionnaires, logs, field notes and emails to evaluate fidelity, dose delivered, dose received, reach and context. Results are presented descriptively. Two authors independently identified key responses that were merged into themes. Although no difference in disability was found between interventions, closer evaluation revealed that participants in PRESTO-Play reported that they learned about prevention of physical complaints and were more satisfied with course contents compared with PRESTO-Fit. Study design and contents of the interventions were found to be valid, with an appropriate dose delivered. Feedback from students and logs suggested that behavioural change and psychosocial principles in PRESTO-Play might have not been implemented optimally. Only moderate fidelity in both groups and too little contrast between interventions could have influenced results. Low attendance rates and a presumed lack of generalization further decreased possible effectiveness. Context greatly influenced implementation. Implementing a future health course with closer collaboration with the institution could optimize accessibility and communication, encourage attendance and enhance motivation for behavioural change.