Check-in/check-out (CICO) is an evidence-based multicomponent intervention widely implemented in elementary and middle schools. The purpose of this study was to evaluate whether an adapted CICO intervention, with CICO components designed to be more developmentally appropriate for adolescents and contextually relevant for high schools, could be implemented with procedural fidelity and improve outcomes for students with and at risk for disabilities. The study included three ninth-grade students (two with Individualized Education Programs) in one U.S. public high school. We used a non-concurrent multiple baseline design conducted over two academic years during the COVID-19 pandemic to experimentally evaluate the effects of CICO-Secondary on student academic engagement. Results demonstrated that students participated with adequate procedural fidelity and that there was a functional relation between implementation of CICO-Secondary and improved student academic engagement. Students and school personnel also perceived CICO-Secondary to be socially valid. Implications for both future research and practice are discussed.