In 2020, the New Zealand Ministry of Education updated "Relationships and sexuality education: A guide for teachers, leaders and Boards of Trustees." Rejecting reductive 'sex education' (which is limited to biology), sexuality education also attends to children's changing sociocultural contexts. The "Guidelines" include a glossary of definitions in dictionary format, including sex, gender and gender identity. These are suggested as key learnings and topics for study across all curriculum subjects. While the dictionary format suggests that 'sex', 'gender' and 'gender identity' have agreed meanings, they are currently objects of bitter dispute across the western world -- in health care, in academia, between activist groups, in sport, and in law reform. The "Guidelines" were released into the crossfire of the global 'gender wars.' How and why have the words 'sex', 'gender' and 'gender identity' recently become so contentious? And how might these debates affect schools? This paper embeds the "Guidelines"' definitions of these terms in international professional, academic and activist debates from the mid-twentieth century to the present. It falls into four parts. The first explores the history of the terms "sex," "gender" and "gender identity." The second, introduces contemporary "transsexual," "queer" and "transgender" academic and activist arguments. Part Three outlines disputes between 'gender critical' feminists and radical transgender activists. In Part Four, I review research on how schools are being affected by the 'gender wars.' The Ministry requires each school to engage in 'consultation with communities' on its approach to the 'sexuality and relationships' curriculum. They have to decide whether to teach, what to teach, when to teach, how to (and how not to) teach these topics and to appraise any outside groups seeking to contribute. Understanding the historical, intellectual, professional and political battles in the 'gender wars' should help in these deliberations.