This study evaluated the effects of a comprehensive school-based sexuality education programme in enhancing sexual health knowledge and understanding among adolescents, parents and school personnel in six secondary schools in Hong Kong. Study outcomes were evaluated through pre- and post-tests. A total of 1588 students, 40 parents and 271 school personnel participated. Baseline student knowledge scores were highest among younger students, with significant improvements observed in both years for all grades except one. Baseline student attitude scores were = 85% for younger students, with no significant improvement, while older students showed significant improvements. Parents scored below 40% correct at baseline for knowledge items, while self-efficacy items were generally higher. Baseline teacher knowledge scores were higher than parents, but self-efficacy scores were comparable or lower. Baseline guidance counsellors' surveys indicated low confidence levels and awareness of community resources to support students. This study identified a clear need for sexuality education in Hong Kong and a special focus on students in schools traditionally serving a high proportion of non-Chinese speaking students. The programme has significant potential for expansion in Hong Kong schools as well as for dissemination in Mainland China.