This study examines whether parenting practices contribute to the improvement of Organization, Time Management and Planning (OTMP) skills following an intervention aimed to support youths with Attention Deficit Hyperactivity Disorder (ADHD) and their parents during their transition to secondary school. Families were randomly assigned to an intervention (n = 32) or a control group (n = 35) and assessed at baseline (T1), post group (T2) and post-intervention (T3). The intervention combines three components (child group, parent group and mentorship). Multivariate regression analysis and test of indirect effects were conducted using PROCESS software. Intervention effects on OTMP skills improvement (T3) was significant both directly, and via parental consistency at T2. The study shows the relevance of targeting consistency in parenting practices at a pivotal moment for children, the transition between primary (Grade 6) to high school (Secondary 1). It also highlights the relevance of directly addressing OTMP skills with ADHD youths.