School-based organization, time management, and planning skills-related (OTMP) interventions have been developed to address academic and organizational difficulties students with Attention-Deficit/Hyperactivity Disorder face (ADHD), especially when entering secondary school (DuPaul et al., 2012; Evans et al., 2018; Villodas et al., 2014). For OTMP interventions to be reliably administered, interventionists must be appropriately trained to not only implement session procedures that adhere to intervention protocol, but to also adjust their responses to individual students to maintain quality interactions, which is referred to as competence (Goense et al., 2016; Perepletchikova et al., 2007). This study tested the hypothesis that the constructs, interventionist competence and student engagement, would significantly change over the course of a 16-session school-based intervention for adolescents with ADHD and academic challenges. Specific student characteristics were also expected to interact with initial levels or changes in competence and engagement over time. Using an archival dataset (N= 111) and latent growth modeling, findings revealed that neither competence nor engagement changed significantly over time. However, initial levels of both constructs significantly varied. Further conditional growth modeling found that greater ADHD symptom severity negatively contributed to competence and that internalizing symptoms contributed uniquely and positively to competence. Although interventionist competence and student engagement did not exhibit significant change over time, certain student factors were associated with the quality of interventionists responses to students and with the degree to which students remain engaged with intervention materials.