Background: Schools are increasingly recognized as key facilitators of child and youth well-being. Much attention has been directed to the school social environment and the areas of school climate or school connectedness/identification. Drawing on the social identity approach and related work, it has been argued that school social identification may be the mechanism or process through which school climate comes to impact individual student functioning ("Applied Psychology," 28, 2009, 171). Much of the previous research on social identity and well-being, though, is limited because it is cross-sectional. Aims, Sample & Methods: This current study aims to advance understanding of the relationships between school climate, school identification and positive and negative well-being. It adopts a three-wave longitudinal sample of Australian students (N = 6537 wave 3, grades 7-10) and incorporates a range of control variables. Multilevel modelling (MLM) is used to test relationships of interest. Results and Conclusions: In line with predictions, school identification was a significant mediator of the relationship between school climate and the well-being dimensions of positive affect and depression (but not anxiety). The substantial theoretical and practical implications of this research are discussed, including the role of the school social environment in helping young people successfully transition to adulthood.