This study evaluates a peer-delivered intervention for high school students with impairing attention deficit hyperactivity disorder (ADHD) symptoms targeting organization, time management, and planning (OTP) and motivation (Students Taking Responsibility and Initiative through Peer Enhanced Support; STRIPES). A mixed methods open trial (Study 1; N = 18) and parallel group randomized controlled trial (Study 2; N = 72) were conducted to examine acceptability, target mechanisms, student outcomes, population fit, and feasibility. Study 1 established acceptability for STRIPES delivered after school but identified forgetfulness and competing social activities as population-specific implementation barriers. In Study 2, three schools employed unique implementation strategies and results varied. An elective pullout model engaging 12th grade peer interventionists under teacher supervision demonstrated good fidelity, attendance, and population fit and significant between-group differences in book bag organization (d = 1.11), academic motivation (d = 0.85 to 2.05), and class attendance (d = 1.47) over time compared to control. When implementation strategy demonstrates population fit, STRIPES shows promise for preventing declining school engagement across ninth grade. [For the corresponding grantee submission, see ED601979.]