The present study reports on the initial validation of the eight-item version of the Avoidance and Fusion Questionnaire for Youth (AFQ-Y8) as a school mental health screener for identifying clinical-level depression and anxiety caseness within a sample of urban high school students (N = 219). Results indicated that responses to the AFQ-Y8 yielded better data–model fit and comparable internal consistency and convergent validity in relation to responses to the longer, 17-item version of the measure. Findings from receiver operating curve (ROC) analyses showed that scores derived from the AFQ-Y8 had excellent discrimination ability for correctly classifying students with and without clinical-level depression (area under the curve [AUC] = .91) and anxiety (AUC = .92), and that a cutoff score of ≥15 yielded optimal sensitivity (.86, .92) and specificity (.88, .87) for accomplishing these purposes. Taken together, findings suggest the AFQ-Y8 is a technically adequate instrument for both measuring psychological inflexibility and classifying students with clinical-level internalizing problems. Implications for future research and practice are discussed.