Social skills training is a common treatment method for adolescents with autism spectrum disorder (ASD), yet very few evidence-based interventions exist to improve social skills for high-functioning adolescents on the spectrum, and even fewer studies have examined the effectiveness of teaching social skills in the classroom. This study examines change in social functioning for adolescents with high-functioning ASD following the implementation of a school-based, teacher-facilitated social skills intervention known as "Program for the Education and Enrichment of Relational Skills" ("PEERS®"). Seventy-three middle school students with ASD along with their parents and teachers participated in the study. Participants were assigned to the "PEERS®" treatment condition or an alternative social skills curriculum. Instruction was provided daily by classroom teachers and teacher aides for 14-weeks. Results reveal that in comparison to an active treatment control group, participants in the "PEERS®" treatment group significantly improved in social functioning in the areas of teacher-reported social responsiveness, social communication, social motivation, social awareness, and decreased autistic mannerisms, with a trend toward improved social cognition on the Social Responsiveness Scale. Adolescent self-reports indicate significant improvement in social skills knowledge and frequency of hosted and invited get-togethers with friends, and parent-reports suggest a decrease in teen social anxiety on the Social Anxiety Scale at a trend level. This research represents one of the few teacher-facilitated treatment intervention studies demonstrating effectiveness in improving the social skills of adolescents with ASD in the classroom: arguably the most natural social setting of all.