School‐based relationships and sexuality education (RSE) offers broad social, emotional and physical health benefits. However, many teachers feel unprepared to deliver comprehensive, contemporary RSE. To address this, the Curtin RSE Project supported schools in implementing whole‐school RSE using a Health Promoting Schools framework, through a supported case study intervention. A multiple, embedded case study design was employed to evaluate implementation in participating case study schools. Of the four schools recruited, data were collected from one primary and one secondary school. This paper reports on student surveys (Grades 6 and 7–12) and focus group discussions and interviews with teachers, parents and students (Grades 9–12). Descriptive statistics were generated for the surveys and reflexive thematic analysis conducted for qualitative data. Students found RSE relevant, though some reported discomfort during sessions. The breadth of RSE topics in the primary school increased post‐intervention. Four themes were reflexively identified to understand RSE implementation: RSE is systematically not prioritised; Approaches to age‐appropriate education; Schools are appropriate environments; and A whole‐school approach for success. Support from school leadership was essential for proper implementation of whole‐school RSE, supported by passionate and well‐supported school staff. Whole‐school approaches ensured consistent messages within the school and home environment. There is a pressing need to embed RSE in pre‐service teacher education and to provide coordinated, ongoing professional learning for in‐service teachers. Provision of technical support for schools facilitates implementation. These strategies will ensure delivery of contemporary, high‐quality RSE and support sustainable whole‐school implementation