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This study examines for whom and how Organizational Skills Training-Tier 2 (OST-T2, Nissley-Tsiopinis et al., 2024) works for students in grades 3 to 5 with organization, time management, and planning (OTMP) difficulties. The study: (1) investigates subgroup differences for the effect of OST-T2 on student homework and academic performance at post-intervention (about 14 weeks), short-term (about 22 weeks) and long-term follow-up (about 55 weeks); and (2) examines whether improvements in student OTMP skills at 14 weeks mediate the effect of OST-T2 on academic and homework performance at 22 weeks. The study used a cluster-randomized design with 22 schools; 182 students in moderation analyses and 133 in mediation analyses. Subgroup-defining variables were: (a) likely diagnosis of ADHD, (b) caregiver-reported child anxiety, (c) severity of baseline OTMP and homework problems, and (d) family socioeconomic level. Findings suggested that OST-T2 is more effective among children with higher anxiety levels as reported by caregivers. The difference in change scores between the high and low anxiety groups on the caregiver-rated metric of OTMP and homework problems was −0.76 (Cohen's d) at post-treatment (95% CI = −0.12, −1.21, p = .021), −0.66 at 22 weeks (95% CI = −0.08, −1.09, p = .017), and − 0.55 at 55 weeks (95% CI = −0.07, −1.03, p = .026). The analyses generally did not detect subgroup effects based on ADHD diagnostic status and families differing in socioeconomic levels. Results also suggested improvements in OTMP skills in response to OST-T2 are likely to signal downstream impact on student homework performance; estimated average causal mediation effect was −0.66 (95% CI = −1.27, −0.19, p < .001), whereas the average direct effect was only −0.07. Additional longitudinal research with larger samples on predefined subgroups is needed.