The Student Engagement Instrument (SEI) is a self-report measure of cognitive and affective engagement with school. Prior SEI validation studies have focused primarily on construct validity through analyses of internal consistency, factor analysis, and measurement invariance. Results are presented here from a two-pronged study of the criterion validity of SEI scores. Using a middle school sample (N = 35,900), concurrent validity was assessed through analyses of group differences in SEI scores across student subgroups expected to differ in cognitive and affective engagement levels: behaviorally disengaged versus non-disengaged, high-risk versus low-risk disability status, and high versus low academic achievement. Next, through multiple logistic regression analyses, the 4-year predictive validity of SEI scores for on-time graduation and dropout was assessed in a cohort of first-time ninth graders (N = 11,588). Nearly all SEI factors demonstrated directionally consistent associations with each criterion, including considerable long-term predictive associations with both dropout and on-time graduation.