Little is known about the nature and mechanisms of factors that facilitate or inhibit the scale-up and subsequent implementation of school-based adolescent sexual and reproductive health (ASRH) interventions. We present process evaluation findings examining the factors that affected the 10-fold scale-up of such an intervention, focussing on teachers' attitudes and experiences. Qualitative interviews and focus group discussions with teachers, head teachers, ward education coordinators and school committees from eight schools took place before, during and after intervention implementation. The results were triangulated with observations of training sessions and training questionnaires. The training was well implemented and led to some key improvements in teachers' ASRH knowledge, attitudes and perceived self-efficacy, with substantial improvements in knowledge about reproductive biology and attitudes towards confidentiality. The trained teachers were more likely to consider ASRH a priority in schools and less likely to link teaching ASRH to the early initiation of sex than non-trained teachers. Facilitating factors included teacher enjoyment, their recognition of training benefits, the participatory teaching techniques, support from local government as well as the structured nature of the intervention. Challenges included differential participation by male and female teachers, limited availability of materials and high turnover of trained teachers.