Background: The benefits of involving intended recipients, implementers and other stakeholders in the co-production of public health interventions are widely promoted. Practical accounts reflecting on the process and value of co-production in intervention design, however, remain scarce. We outline our approach to the co-production of two multi-component, school-based relationships and sex education interventions. We reflect on the utility of involving school staff, students, and other youth, professional and policy stakeholders in intervention design and on some of the challenges we encountered during the process. Methods: Seven consultations were conducted in southeast and southwest England involving 75 students aged 13–15 and 22 school staff. A group of young people trained to advise on public health research were consulted on three occasions. Twenty-three sexual health and sex education practitioners and policy makers shared their views at a stakeholder event. Written summaries of activities were prepared by researchers and shared with the specialist provider agencies for each intervention. Negotiated consensus between researchers and providers was reached about how participant views should inform intervention content, format and delivery models. Results: Consultations confirmed acceptability of intervention aims, components and delivery models, including curriculum delivery by teachers. They sensitised us to the need to ensure content reflected the reality of young people’s experiences; include flexibility for the timetabling of lessons; and to develop prescriptive teaching materials and robust school engagement strategies to reflect shrinking capacity for schools to implement public-health interventions. Accessing and prioritising stakeholder feedback was not always straightforward, however, where specific expertise or capacity for participation was limited or when participant views contradicted best practice, budget or the randomised controlled trial design. Conclusions: Involving potential recipients, implementers and wider stakeholders as co-producers in intervention design can bring valuable insights that can help reduce research waste. Successful co-production can be complex and challenging and requires careful consideration of the topics participants can most usefully speak to; the representativeness of those involved; the capacity available for participation; and how participants will be compensated. Findings also alert us to the importance of having well-defined, transparent procedures for deciding how stakeholder input will be incorporated.