BACKGROUND: Whole-school interventions represent promising approaches to promoting adolescent sexual health, but they have not been rigorously trialled in the UK and it is unclear if such interventions are feasible for delivery in English secondary schools. The importance of involving intended beneficiaries, implementers and other key stakeholders in the co-production of such complex interventions prior to costly implementation and evaluation studies is widely recognised. However, practical accounts of such processes remain scarce. We report on co-production with specialist providers, students, school staff, and other practice and policy professionals of two new whole-school sexual heath interventions for implementation in English secondary schools. METHODS: Formative qualitative inquiry involving 75 students aged 13-15 and 23 school staff. A group of young people trained to advise on public health research were consulted on three occasions. Twenty-three practitioners and policy-makers shared their views at a stakeholder event. Detailed written summaries of workshops and events were prepared and key themes identified to inform the design of each intervention. RESULTS: Data confirmed acceptability of addressing unintended teenage pregnancy, sexual health and dating and relationships violence via multi-component whole-school interventions and of curriculum delivery by teachers (providing appropriate teacher selection). The need to enable flexibility for the timetabling of lessons and mode of parent communication; ensure content reflected the reality of young people's lives; and develop prescriptive teaching materials and robust school engagement strategies to reflect shrinking capacity for schools to implement public-health interventions were also highlighted and informed intervention refinements. Our research further points to some of the challenges and tensions involved in co-production where stakeholder capacity may be limited or their input may conflict with the logic of interventions or what is practicable within the constraints of a trial. CONCLUSIONS: Multi-component, whole-school approaches to addressing sexual health that involve teacher delivered curriculum may be feasible for implementation in English secondary schools. They must be adaptable to individual school settings; involve careful teacher selection; limit additional burden on staff; and accurately reflect the realities of young people's lives. Co-production can reduce research waste and may be particularly useful for developing complex interventions, like whole-school sexual health interventions, that must be adaptable to varying institutional contexts and address needs that change rapidly. When co-producing, potential limitations in relation to the representativeness of participants, the 'depth' of engagement necessary as well as the burden on participants and how they will be recompensed must be carefully considered. Having well-defined, transparent procedures for incorporating stakeholder input from the outset are also essential. Formal feasibility testing of both co-produced interventions in English secondary schools via cluster RCT is warranted. TRIAL REGISTRATION: Project Respect: ISRCTN12524938 . Positive Choices: ISRCTN65324176.