This paper elaborates possibilities and limitations for sex education in schools by mapping the consequence of introducing the topic 'sex' within a range of scientific and informal discursive settings. The organising and regulating structures of the school maintain the boundaries between common-sense and scientific discourses. Bernstein made a distinction between vertical discourse and horizontal discourses. Vertical discourses characterise scientific discourses that are coherent, explicit and systematically principled structures. Horizontal discourses characterise common-sense discourses that are local, segmented and context-dependent. The implications for teaching sex education through vertical discourses such as science, English and art as opposed to personal and social education, which is a horizontal discourse, are discussed. Yet, all discourses are unstable, and the notion of a 'tipping point' is introduced to demonstrate the potential for sex education when vertical discourses collapse.