This paper presents school climate assemblies, conceived as an innovative educational tool for the co-creation of climate change and sustainability solutions in educational institutions and communities. It describes an intervention research project that builds on previous processes and experiences of climate citizen assemblies which have been organised in different countries across the world. School climate assemblies are conceived of as a tool that promotes the active engagement of pupils and their communities in co-creating climate change mitigation and adaptation strategies towards jointly defining innovative sustainability and climate change solutions. An intervention, piloted in five secondary schools in the Spanish region of Tarragona, is reported, presenting the main phases and steps followed. To test the effectiveness of the intervention, this paper outlines the policy climate change recommendations that emerged from the co-creation process to identify and agree on a set of actions to tackle climate change and sustainability challenges and the degree of student satisfaction with the intervention. A detailed methodology for organising school climate assemblies that can be replicated in other educational institutions, contexts and regions, is presented, enabling the widespread adoption of this innovative approach. The study emphasises the potential of co-creation methods in the educational context and underscores the critical role of education in fostering sustainability in citizens and empowering students to become active change agents in their local communities. The results lead to policy implications for integrating co-creation approaches into school curricula worldwide, aiming to empower students and drive sustainability action.