Comprehensive sex education (CSE) has been heralded as effective in promoting sexually healthy behaviour in youth. At the same time, it has also been countered by critique, indicating that CSE is not a neutral vehicle for the transmission of knowledge. To think sex education outside this opposition of health intervention and critique, this article asks: What else can sex education do? Three ethnographic cases from secondary schools in the Netherlands showed the school to be a space/time for sexuality, showed how sexual knowledge is produced and used in class, and how sex education plays into and depends on processes of (gendered) popularity. In addition, the analysis pointed to the ways in which comprehensive sex education in practice (re)produces ethnic characterizations of sexuality. Finally, the analysis of sex education in practice complicated the ways in which sex education is conceptualized and measured as a health intervention.