Small scale trials indicate that classroom-based Cognitive Behaviour Therapy (CBT) for adolescents has good reach and can help prevent depression. However, under more diverse everyday conditions, such programmes tend not to show such positive effects. This study examined the process of implementing a classroom-based CBT depression prevention programme as part of a large (n = 5,030) randomised controlled trial across eight UK secondary schools which was not found to be effective (PROMISE, ISRCTN19083628). The views of young people (n = 42), teachers (n = 12) and facilitators (n = 16) involved in the Resourceful Adolescent Programme (RAP) were obtained via focus groups and interviews which were thematically analysed. The programme was considered to be well structured and contain useful content, particularly for younger pupils. However, challenges associated with implementation were its age appropriateness for all year groups, its perceived lack of flexibility, the consistency of quality of delivery, the competing demands for teacher time and a culture where academic targets were prioritised over personal, social and health education. Whilst schools are convenient locations for introducing such programmes and allow good reach, the culture around improving well-being of young people in schools, increasing engagement with teachers and young people and sustaining such programmes are issues that need addressing.