Externalizing problems are common in children ages 6-14, can have lifelong consequences, and may pose a particular risk when combined with other risk factors and symptoms (like depression and anxiety). Schools are uniquely positioned to assess and address these types of behavioral health concerns, but many school-based assessments do not focus on mental health distress (partially because they often lack the infrastructure for identification, screening, and referral). To address this gap, the Behavioral Health Works program student mental health software system has integrated teacher training, psychometrically strong assessments, feedback, and referral tools. However, this self-report tool for adolescents needed to be adapted for younger children. Thus, a parent-report version was added as well as new scales for better assessing this age group. The present study examines the psychometric properties of the new parent-report attention-deficit hyperactivity disorder (ADHD) and oppositional defiant/conduct scales within a sample of 440 children referred for school-based assessments. Overall, the new scales demonstrated good structural validity, measurement invariance across most demographic groups, discrimination in item response theory analyses, and evidence of convergent validity and good classification accuracy in relation to a validation battery. These externalizing scales are distinct and precise and show promise for improving the effectiveness of school-based programs for identifying at-risk children.