Girls' vulnerability to sexual violence and harassment is a recurrent theme in much of the literature on schooling in sub-Saharan Africa. Within this research, girls are often framed as passive victims of violence. By drawing on a case study, this paper focuses on 12 to 13-year-old South African school girls as they mediate and participate in heterosexual cultures that are simultaneously privileging and damaging. Set against the wider social context where violent gender relations are key to the building blocks of patriarchy, the paper examines how heterosexuality underscores the formation of femininity as girls engage with and participate with each other and boys in informal school relations. To this end, Butler's concept of the "heterosexual matrix" is deployed to examine how girls navigate the wall of male power, where the "real" expression of femininity is embedded within heterosexuality. The paper explores girls' investment in heterosexual cultures in the school playground and on "dress-up Friday" to examine how gender power inequalities and violent relations manifest. In expanding the analysis of heterosexuality to primary school contexts, the paper broadens the focus of school-based gender and sexualities research in sub-Saharan Africa to address a neglected area of younger girls' femininity and their active agency. The paper argues for the importance of addressing primary school girls, femininity and the power of heterosexuality through which relations of inequalities operate.