We assess the impact on student knowledge, attitudes and behavior of a typical HIV-AIDS teacher training program, funded by the African Development Bank and implemented with technical assistance from UNESCO in Cameroon. Applying an identification strategy based on instrumental variables that controls for teacher selfselection into the training program, we find that exposure to a trained teacher increases the apprehension of 12 to 13 year olds concerning HIV-AIDS, by rendering them less likely to be willing to buy from an HIV-positive shopkeeper, and more likely to wish to remain sexually abstinent during adolescence. No effect is found either on their knowledge or on their behavior. For 16 to 17 year olds, on the other hand, exposure to a trained teacher increases the likelihood that students are willing to discuss HIV-AIDS issues within their families. More importantly, older students who are exposed to a trained teacher are 29% more likely to have used a condom during their last sexual intercourse, and are 27% more likely to have carried out an HIV test.