Students with emotional-behavioral disorders are particularly vulnerable to adverse mental health consequences as they often experience less school success than any other group of students, with or without disability (Beaudoin et al., 2008; Wagner et al., 2005). Existing research indicates that students who participated in social-emotional learning programs, such as Second Step, demonstrated significant improvement with social and emotional skills, behavior, and academic performance, than students who did not (Durlak et al., 2011). However, less is known about the science related to systems-level implementation to support applied practices (Fixsen et al., 2005), and little is known about implementing SEL programs in alternative settings that support students with social and emotional difficulties. This retrospective case study examined the feasibility of implementing the Second Step curriculum, and a social-emotional competence assessment system, the Devereux Student Strengths Assessment (DESSA), within a clinical day treatment program. This study focused on the implementation process to evaluate if and how Second Step could work in this this alternative setting. This study used thematic analysis within a mixed-methods explanatory sequential design, comprised of two phases including the collection and analysis of quantitative data and qualitative data. Lessons learned from this case study will be influential for determining the best ways to strengthen the implementation process of social-emotional learning interventions, particularly in clinical day treatment settings.