School-based sex education is an underdeveloped and challenging issue to address in Iran. This paper provides insights into the main challenges in developing and implementing school-based sex education in Iran. Through an investigation of one Iranian boys' school that, in contrast to the majority of Iranian educational institutions, has an established strategy for educating its pupils on sexual matters, it is possible to critically explore the underlying principles and practice of sex education in this institution. Given the sociocultural milieu provided by Iranian society and the problematic nature of childhood, adolescent and premarital sexualities within that society, developing and implementing school-based sex education programmes is a difficult endeavour. Premarital sexuality in Iran is unacceptable to such a great extent that many parents and schools, even when aware of the risks associated with poor sexual health knowledge, may choose not to approach sex education, since to do so may be to acknowledge premarital sexuality and, supposedly, lead to premarital sex. There is a need for evidence-based research to examine the different aspects of sex education for young Iranians and to demonstrate its importance and effectiveness.