In this paper we seek to empirically quantify the role of peer social networks in influencing sexual behavior among adolescents. Using data of a nationally representative sample of adolescents we utilize a multivariate structural model with school-level fixed effects to account for the problems of contextual effects, correlated effects and peer selections to purge the potential biases from the estimates of peer influence. Our peer group measures are drawn not only from the nomination of close friends, but also from classmates. Controlling for parent level characteristics, and other demographic parameters, we find that a 10% increase in the proportion of close friends who initiates sex increases the probability that an individual chooses to initiate sex by 5% and a 10% increase in number of sexual partners among close friends increases an individual's sexual partner by 5%. The influence of classmates however, diminishes in magnitude after accounting for unobserved environmental confounders. (Contains 4 tables.)