Teenagers are often disengaged in high school. Self-determination theory suggests that support for competence, autonomy, and relatedness may facilitate engagement. Co-creation encourages students to take responsibility for their affective engagement and work with teachers to create a context that supports their basic needs. Youth Participatory Action Research (YPAR) may support co-creation by providing a structure to support student agency in resolving issues that are important and valued by the students. A teacher-researcher conducted this action research dissertation to explore how to engage students in an elective Peer Mediation class. The research questions in this study were: 1) How does co-creation support student engagement, and 2) in what ways does YPAR support co-creation? Students formed six YPAR groups. Data from student assignments, conversations, and surveys measuring academic-achievement-emotions and basic psychological needs were used to gauge student affective engagement throughout the semester. Students also reflected on their experiences and provided feedback to enrich the class. Findings suggest students were engaged when they experienced competence, autonomy, and relatedness. However, it was a challenge to help students see themselves with agency, capable of ensuring their needs were satisfied and creating change in the school community. Quantitative results demonstrated a statistically significant increase in relatedness. Qualitative data revealed that students developed competency, experienced autonomy, and built relationships that sustained engagement. YPAR and co-creation provided opportunities for students to take responsibility for their feelings and meet their needs. YPAR and Peer Mediation may be a powerful match because they share the purposes of developing awareness, solving meaningful problems, and promoting social justice. [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.]