Schools are ideal locations for youth to receive mental health and social-emotional programming. Despite the well-established need for school-based mental health programs, many schools do not offer these programs, and those that do often face implementation challenges that hinder the effectiveness of the programs. Peer-led programs may be an effective way to implement school-based mental health prevention efforts. Peer-led programs employ influential student leaders trained to deliver prevention content and serve as role models to their peers. Understanding potential barriers to the implementation of school-based programs is critical to ensure these programs have a meaningful impact on students and schools. The current qualitative phenomenological study examines the lessons learned from implementing the "Sources of Strength" ("Sources") prevention program in 11 high schools in a western US state. "Sources" is an evidence-based peer-led suicide prevention program focused on developing protective factors and promoting resilience in schools. Implementing the "Sources" program was successful in many ways, including facilitating positive student-staff interactions, cultivating a welcoming and inclusive environment in the school community, and encouraging community outreach. Barriers to implementation were also found, including time constraints, low peer leader engagement, inefficient communication between peer leaders, need for additional guidance from program staff, and school buy-in. The findings provide program developers and schools with strategies to help strengthen program implementation and highlight challenges that "Sources" and similar peer-led programs face.