Age-appropriate use of social skills throughout a student's K-12 career has been linked to a variety of favorable academic and behavioral outcomes (Capara et al., 2000; Malecki & Elliot, 2022; Wentzel, 1993). Social skill deficits are widely present in the secondary setting (Gresham et al., 2010), and research has highlighted 9th grade as an opportune time to address social skills due to the transitional nature of this year (Akos & Galassi, 2004; Isakson & Jarvis, 1999; January et al., 2011; Wentzel et al., 2019). Further, certain subgroups of students, including students with disabilities and those at-risk for social, emotional, and behavioral (SEB) challenges, are at increased risk of having social skill deficits (Fussell et al., 2005; Griswold & Townsend, 2012; Rose et al., 2011, Smith & Wallace, 2011). This study employed a pre- and post-test control group design within one high school to assess the effectiveness of universal social skill lessons targeted for 9th grade students on teacher and student reports of social competence. Results from this study indicate the intervention was effective for 9th grade students overall, as well as for students identified as at-risk for SEB challenges according to teacher report. Additionally, teacher report data showed promise for the intervention's effectiveness for the subgroup of students with disabilities. Alternatively, all analyses of student report data were not significant. Following the intervention, teachers found the intervention acceptable, feasible to implement, and important for their students. Implications and future research are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]