Schools in the UK and Europe, North America and Australia are introducing ambitious forms of relationships and sex education (RSE) or school-based sexuality education. For RSE to be effective it must be inclusive, recognising and respecting the needs and experiences of those who have not always been well served by sex/sexuality education. This paper considers one such group -- students with faith backgrounds -- and explores ways of delivering RSE in the 'postsecular classroom' in which religion is recognised and respected. We conducted consultative research -- designed primarily to inform the development of teaching resources -- among students and parents of faith, and RSE teachers. Focussing upon two religiously diverse cities in England, this research included systematic literature review, classroom observations and group discussions with students, and questionnaire surveys and interviews with parents and teaching staff. Informed by the findings of this research, we designed, piloted and now share evidence-based teaching resources. This illustrates one way in which RSE can be adapted for use in the postsecular classroom where faith is out in the open, but not necessarily explicitly engaged with in the lesson. Considering the perspectives of faith communities in this way can improve RSE for everyone in the classroom.