The development of Social-Emotional Learning competencies can have a profound impact on student success, helping students build connections to their school, teachers, and peers, overcome challenges, and develop academic habits that foster academic and personal success. In high schools SEL skills are often taught in isolation from academic subjects, partitioned off to Advisory classes or school-wide assemblies, or viewed as the domain of student support staff and not classroom teachers. This study examined the impact of embedding SEL instruction in core subject lesson plans, evaluating the impact of classroom-embedded SEL instruction on students' knowledge of and application of SEL competencies; student sense of school connectedness and belonging; and students' academic success. Teachers received in-service training on SEL instruction and worked collaboratively to develop lessons that integrated the SEL competencies of Self-Management, Responsible Decision-Making, and Social Awareness into their lessons. Students in these teacher's classes were administered surveys in the fall of the 2021-2022 school year to assess knowledge of SEL competencies, as well as a school climate survey to measure connectedness to school and feelings of belonging. Following classroom-embedded instruction, students took a post-survey in SEL competencies and school climate and participated in focus groups to evaluate the impact of the lessons. Data were also collected on student grades, attendance, and referral to the Student Support Team, as well as lesson documents. [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://www.proquest.com/en-US/products/dissertations/individuals.shtml.]