Procrastination in academic contexts is thought to have a negative effect on students' learning and performance. This research sought to provide a comprehensive multi-method and multimodal validation of a self-report measure of procrastination, revealing its intricate associations with behavioral indicators of procrastination, engagement, and learning. A sample of 566 high school Advanced Placement (AP) Statistics students (mean age = 16.68 years) in the midwestern United States participated in the investigation. We used multiple methods and multiple traits to evaluate convergent and discriminant validity evidence. To gather evidence of convergent validity, we examined associations between self-reported procrastination and actual procrastination behavior extracted from log data using survival analysis. Students were much more likely to not submit assignments on time when self-reported academic procrastination was high. This trend persisted across all assignments, underscoring the robust link between self-reported procrastination and delayed completion. The results also highlighted strong significant negative correlations between self-reported academic procrastination and self-reported affective, behavioral, and cognitive engagement. To examine predictive validity, we also examined the association between self-reported academic procrastination and key learning outcomes. While self-reported procrastination was not significantly associated with students' AP exam scores, it was significantly and negatively associated with students' final class grades. Students with greater self-reported procrastination tended to have lower class grades, consistent with prior work linking procrastination to suboptimal learning outcomes. These findings could inform educational interventions that reduce procrastination tendencies, enhance student engagement, and ultimately, academic success. [This paper will be published in "Translational Issues in Psychological Science."]