This case-study analyses the experiences of a trans pupil and teacher in a UK- secondary school. It aims to better understand cultural cisgenderism in schools, the acceptance -- or lack thereof -- of gender expansiveness, and the extent to which school policies are useful to challenge cisnormative gender regimes. Semi-structured interviews were conducted with the trans pupil and teacher and were analysed thematically. Document analysis of the school's equality and diversity policy was also conducted. Findings revealed that cultural cisgenderism and cisnormativity are embedded into the school culture, with gender structures evident in several areas, leading to trans marginalisation. There was also evidence that both teachers and pupils may contribute to exclusionary and reactive environments, where accommodations are provided only when a trans pupil is visible. This research may be useful for those working in schools to critique policies and practices and create positive change for trans pupils.