Supporting transgender and gender-nonconforming youth in schools involves changes at all levels of education. Gender-complex education, or education that takes into consideration the existence and experiences of transgender and gender-nonconforming people, should be a basic and pervasive part of curricula and should be seen as critical for students of all gender identities and presentations; it must be infused within every content area, including mathematics. This article synthesizes perspectives on gender-complex education, teaching mathematics for social justice, and research on students' development of proportional reasoning and statistical concepts, and then proposes a mathematics project for middle schoolers to facilitate their agency in challenging transphobia and gender oppression in their schools.