Preventing sexual risk behaviors among early adolescents is essential, as it may delay premature sexual initiation and reduce the incidence of sexually transmitted infections. This quasi-experimental study aimed to evaluate the effectiveness of a sexual health literacy promotion program in enhancing preventive sexual risk behaviors among lower secondary school students. The sample consisted of 114 eighth-grade students selected through multistage sampling and assigned to either an intervention group or a control group, with 57 participants in each group. The intervention group received the sexual health literacy promotion program (CoMMIT program), based on the health literacy framework and comprising five structured learning activities. The control group received the standard curriculum. Data were collected using demographic, sexual health literacy, and sexual risk behavior prevention questionnaires. Measurements were conducted at pre-intervention, post-intervention, and one-month follow-up. Data were analyzed using two-way mixed-design repeated measures ANOVA and independent t-tests. The intervention group demonstrated significantly higher mean scores for sexual health literacy and preventive sexual risk behaviors at post-intervention and one-month follow-up compared with baseline and the control group (p < 0.001). Large effect sizes were observed for sexual health literacy (η² = 0.932) and sexual risk behaviors prevention (η² = 0.791). The CoMMIT program effectively improved sexual health literacy and preventive sexual risk behaviors among lower secondary school students and is recommended for implementation in secondary schools to promote adolescent sexual health.