The purpose of this study was to provide initial evidence of the effectiveness of Check-In/Check-Out-High School (CICO-HS) on high school student outcomes. Check-In/Check-Out-High School is a version of CICO, an established Tier 2 intervention designed to improve student academic and social behavior, adapted to increase effectiveness and feasibility in high schools. Key adaptations in CICO-HS include increased student agency through goal setting and self-rating, and increased efficiency through use of a mobile application for data collection, progress monitoring, and family communication. This randomized controlled trial (RCT) was conducted during the 2021-2022 school year and included 14 ninth grade students in one public high school in the U.S. Pacific Northwest. Students were randomly assigned to receive CICO-HS or a waitlist/control condition. Although underpowered, the RCT results showed meaningful improvements (small to moderate effect sizes) in student attendance (unexcused tardies and absences) and teacher ratings of student academic and social behaviors. Moreover, students and school personnel rated CICO-HS components as being socially acceptable to implement in high school. Implications for future research to replicate and expand study findings and for practice to scale up CICO-HS with fidelity in high schools are discussed.