Decades of research have shown a normative decline in academic performance to be associated with the transition from elementary school to middle school. Based on the idea that these difficulties stem from a lack of relevant coping skills, the current study focused on the preventive effects of a three-year social and emotional learning (SEL) program in mitigating transitional achievement loss. Quality of implementation, a crucial, but often overlooked, factor in program evaluation, is the framework through which students' intervention experiences were defined. In each intervention year, implementation was assessed through teacher-reported curriculum fidelity and teacher's perception of program quality. These factors were tested as predictors of changes in GPA and standardized test scores across the transition. Intervention dosage received over the fifth grade year emerged as a significant predictor of GPA change. Dosage was unrelated to standardized test change, though differences between genders and ethnic groups in transitional standardized test performance were found. Teachers' ratings of program effectiveness were also unrelated to outcome, but were associated with intervention dosage.