This study evaluates a peer-delivered intervention for high school students with impairing 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 bookbag 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. [This paper will be published in "School Psychology Review."]