Research Findings: This study provides psychometric validation of the Classroom Engagement Scale (CES), a 14-item, teacher-report, formative assessment of behavioral engagement. It is currently used for all kindergarten children in a large, urban school district where it is embedded in the report card. Multi-level factor analysis revealed two distinct dimensions of the measure -- academic engagement and social engagement -- that comport with the educational research literature. Measurement invariance investigations provided initial support for the applicability of the CES across student subgroups and time. Finally, the two dimensions of the CES were found to be differentially predictive of important kindergarten and third-grade outcomes, providing evidence of concurrent and predictive validity. Practice or Policy: This study was designed in response to both the national call for routine monitoring of critical engagement competencies and the lack of validated teacher-report measures of these skills that can be used to inform ongoing instruction. This initial validation is an important step in institutionalizing practical, quality measurement within schools. Because of its strategic placement on the kindergarten report card, the CES provides an opportunity for teachers and families to share information about students' engagement skills and coordinate confluent supports to foster these critical competencies.