This paper is a critical discourse analysis of teacher resource materials about girl bullying. The "mean girl" phenomenon has been widely taken up as one of the current key narratives about schools and school girls. This paper argues for the importance of understanding the origins of this discourse within behavioural psychology, which coined the term "relational aggression" to describe what is purported to be a uniquely female form of aggression. Our analysis problematises the ways in which this discourse reinforces gendered binaries and pathologises girls' aggression. We trace the emergence of an "expert culture," exemplified by the texts we analyse, which disseminates this discourse and limits the range of ways in which girls' aggression and friendships may be understood. We critique the raced and classed ways in which these texts establish hegemonic forms of white, middle-class girlhood. We show how the texts draw on neo-liberal ideologies of idealised forms of subjectivity and function as a mechanism for shaping these subjectivities within schools.