Sedentary behaviour is associated with various health outcomes in adolescence, including unfavourable weight status, reduced physical fitness, self-esteem and school performance, and these associations are largely independent from moderate-to vigorous physical activity. Adolescents spend on average more than 60% of the day (waking hours) sedentary, suggesting effective interventions are needed to reduce sedentary behaviour. As a lot of time in school is spent in a sedentary way, this project focuses on developing, implementing and evaluating interventions to reduce sedentary behaviour in the school setting. A first possible intervention strategy that has gained attention over the last few years is to implement standing desks in the classroom. This strategy is warranted because of the health benefits associated with more standing time in class. However, before this strategy is adopted it is crucial to determine that if it also provides cognitive function benefits and that it is not detrimental to academic performance. Therefore, in a first study, the implementation of standing desks (i.e. a specific intervention strategy) in secondary schools will be evaluated via a clustered randomized controlled trial using an intervention and control condition. The effects on adolescents’ cognitive function (i.e. memory, executive function and attention) and objectively measured sitting and standing time will be investigated. A second study will apply a participatory or co-creational approach by actively involving the adolescents in the development, implementation and evaluation of an intervention to reduce sedentary behaviour at school (including a range of strategies). Adolescents are considered to be co-researchers and co-owners of the project, experiencing elevated levels of empowerment. In the intervention schools, a researcher will set up an action group with adolescents to co-create an intervention, tailored to the needs and interests of the target group. The intervention will then be implemented, in cooperation with the adolescents. Intervention effects on sedentary behaviour and related determinants (e.g. attitude, self-efficacy) will be evaluated via a clustered randomized controlled trial using an intervention and control condition and a pre- and post-test design. Both objective measures and questionnaires will be used. To maximize adolescents’ participation in the evaluation part, adolescents will be able to add questions to the questionnaires, based on what they want to accomplish with the intervention. A process evaluation will be conducted to evaluate how adolescents in the action group experience the participatory process, and to evaluate how other adolescents experience an intervention developed by their peers.