This paper explores schools' new role in promoting children's mental health, as a key focus for policy makers across the global north. An education policy analysis is conducted for England and Australia, two nations advocating a 'bottom-up' approach to mental health promotion, granting flexibility to schools and municipal authorities. Here it is argued that a common policy lexicon is evident where key concepts--wellbeing, resilience, character--are constructed on taken-for-granted assumptions. These are argued to be limited by an emphasis upon the individual constituents of mental health, which is contrasted against a broader conception of wellbeing, evident in recent non-governmental international policy. Empirical data is then presented from two separate studies in England and Australia, where young peoples' perspectives are used to arbitrate the efficacy of the policy construction of wellbeing, in canvassing a relational and social identity approach as a viable alternative. Both studies privilege the voices of young people on wellbeing and identity, with findings from England highlighting that the dominant schooling narrative of resilience perceived by students, misses the ontological and social dimensions of their wellbeing. Research with Indigenous young people in Australia is then evaluated for the affordances generated by identity building and affirming programmes in schools. Notwithstanding cultural differences, common aspects were evident across both datasets, revealing the centrality to young people of self-authenticity, relatedness, and connectedness to nature, as key to their wellbeing. The paper concludes in advancing a set of principles to underpin a relational and social identity approach to schools' wellbeing promotion strategy.