Background: Anxiety and depression are common in children and adolescents, which can be detected via self-report questionnaires in non-clinical settings like the school environment. Two short versions of the Revised Child Anxiety and Depression Scale (i.e., RCADS-25 and RCADS-20) seem to be feasible for administering at schools. The present study evaluated the psychometric properties of the RCADS-25 and RCADS-20 used as screening instruments for anxiety and depression in a general population of schoolchildren and adolescents. Methods: The RCADS-25 was completed by 69,487 schoolchildren and adolescents aged 8 to 18. The RCADS-25 and RCADS-20 broad anxiety scales are equal (15 items), but there are two versions of the major depressive disorder (MDD) scale: the RCADS-25 MDD scale (10 items) and the RCADS-20 MDD scale (5 items). The three scales were assessed on structural validity, internal consistency, test-retest reliability, criterion validity, and hypotheses for construct validity. Results: The RCADS-25/RCADS-20 broad anxiety scale demonstrated a sufficient structural validity (CFI = 0.98, TLI = 0.99, RMSEA = 0.03, SRMR = 0.03), internal consistency (alpha = 0.82), test-retest reliability (ICC = 0.73), criterion validity (AUC = 0.79), and all four hypotheses concerning construct validity were confirmed. The RCADS-25 MDD scale demonstrated a sufficient test-retest reliability (ICC = 0.70) and three out of four hypotheses concerning construct validity were confirmed, but its structural validity was suspect (CFI = 0.89, TLI = 0.94, RMSEA = 0.09, SRMR = 0.06). The RCADS-20 MDD scale demonstrated a sufficient structural validity (CFI = 0.97, TLI = 0.97, RMSEA = 0.08, SRMR = 0.04) and internal consistency (alpha = 0.72). Two out of four hypotheses concerning construct validity were confirmed. The test-retest reliability (ICC = 0.60) was insufficient. Since both MDD scales showed shortcomings, the shortening of the RCADS-25 MDD scale was re-examined post hoc by principal component and reliability analyses. The result was an MDD scale with seven items. Conclusions: The RCADS-25/RCADS-20 broad anxiety scale is valid and reliable for screening schoolchildren and adolescents, but the RCADS-25 and RCADS-20 MDD scales showed shortcomings. An MDD scale of seven items showed acceptable psychometric properties.