Background: Although positive youth development (PYD) programs have demonstrated effectiveness in improving adolescent reproductive health outcomes, there is a lack of evidence on effective school-based interventions designed especially for high school settings. This study examined the efficacy of Peer Group Connection (PGC-HS), a school-based PYD program, in improving sexual health outcomes for high school participants. Methods: A total of 1523 ninth-grade students at 18 schools were randomly assigned to be offered PGC-HS or a classes-as-usual control condition during 2016 to 2017 and 2017 to 2018 school years. Impacts were assessed on three confirmatory and 6 exploratory outcomes via self-reported participant questionnaire data collected at the beginning of 10th grade. Results: Although the offer of PGC-HS had no statistically detectable effect on confirmatory behavioral outcomes (sexual initiation, frequency of sex, and number of sexual partners) at 10th grade follow-up, causal impact estimates indicate that PGC-HS participants were less likely than control participants to ever have had vaginal sex. PGC-HS participants also scored higher on decision-making skills and perceived connection to peer connectedness. Conclusions: Results suggest that by building social and emotional skills and helping students form supportive peer relationships, PGC-HS may encourage students to make healthier choices and avoid risky behaviors during a critical period in high school, thus, reducing the risk of pregnancy.