Addressing the complex homework and organization problems faced by students with attention-deficit/hyperactivity disorder (ADHD) must be balanced with ensuring that interventions are feasible to implement. The Homework, Organization, and Planning Skills (HOPS) intervention was developed to be a brief intervention implemented in typical school settings with minimal training/support required for school clinicians. Given this, it is critical to explore how clinician adherence and competency relate to student outcomes. Participants included 107 adolescents with ADHD ("M"age = 11.95, SD = 1.04; 69% male; 57% on ADHD medication) who participated in the HOPS intervention. Path analyses examined the relation between clinician adherence (fidelity to session content, session length) and competency (e.g., empathic, enthusiastic, calm, collaborative), controlling for baseline scores of the outcome measure and other relevant therapeutic processes (parent and adolescent engagement, working alliance). Clinician fidelity and competency were inversely related, such that highly competent clinicians had lower fidelity to the HOPS content. Importantly, clinician competency was a significant predictor of fewer parent-reported adolescent organizational problems and a higher percentage of assignments turned in post-HOPS, whereas clinician fidelity was unrelated to treatment outcomes. Session length was inversely related to parent-reported homework performance, such that longer session length was associated with worse homework outcomes. Findings suggest that school clinician competency is an important factor in predicting positive treatment outcomes for youth who participate in school-based homework and organization interventions. Training to help school clinicians improve their competency including being more empathetic, calm, motivating, and collaborative with students, and to help them flexibly implement school-based interventions is warranted.