Objectives: To investigate the effects of a service-learning-based health promotion elective in influencing knowledge of diabetes mellitus (DM) and ways to prevent it. Method: A computer-based quiz, an information poster, interactive models and a take-home information leaflet on DM were developed as part of an exhibit during the 2009 National Festival of Science and Technology held in Grahamstown, South Africa. Predominantly school students visited the exhibit and took part in the quiz and other educational activities. Results: The majority of the 119 junior and 332 senior quiz participants were male students attending government schools in the Eastern Cape, South Africa. After an educational intervention, there was significant improvement in the junior quiz participants’ overall percentage knowledge scores (p-value = 0.024), while the senior quiz participants showed a markedly significant improvement in the overall percentage knowledge scores (p-value < 0.001). The results showed significant gender differences for both the pre- and post-intervention mean percentage scores among both groups, with better scores for female participants. Conclusion: The health-promotion elective was successful in raising awareness of DM. This approach may offer an additional tool that can be used in the continuous, concerted health promotion activities and advocacy by all healthcare professionals to address the prevention of DM.