Executive functions (EFs) show promise as important mediators of adolescent academic performance. However, the expense of measuring EFs accurately has restricted most field-based research on them to smaller, non-longitudinal studies of homogeneous populations with specific diagnoses. We therefore monitored the development of 259 diverse, at-risk students' EFs as they progressed from 6th through 12th grade. Teachers completed the Behavior Rating Inventory of Executive Function (BRIEF) for a random subset of their students. At that same time, those same students completed the Behavior Rating Inventory of Executive Function-Self Report (BRIEF-SR) about themselves; teachers generally reported stronger EFs in students than students reported in themselves. Results further indicated that both BRIEF and BRIEF-SR Global Executive Composite (GEC) scores--measures of overall executive functioning--significantly predicted overall GPAs more than was already predicted by students' gender, IEP status, and eligibility for free/reduced school lunch. BRIEF (teacher) scores were better predictors and contributed more to predictive accuracy than the BRIEF-SR (student) scores; BRIEF scores even added additional predictiveness to a model already containing BRIEF-SR scores, while the reverse did not hold. This study provides evidence for valid use of BRIEF and BRIEF-SR GEC scores to predict middle and high school GPAs, thereby supporting practitioners use for this purpose within similar, diverse, at-risk populations. The study also illuminates some of the EF development for this population during adolescence.