This article focuses on the psychometric properties and characteristics of the Assessing School Settings: Interactions of Students and Teachers (ASSIST), an observational assessment administered by trained external observers of teacher practices, classroom context, and student behaviors at the classroom level. Study 1 examines variability, reliability, and convergence between ASSIST scores with data from 3,298 classrooms nested within 185 elementary, middle, and high schools. We report school-level intraclass correlations (ICCs), standard deviations, means, and reliability estimates of ASSIST scales; investigate the correspondence among ASSIST-measured constructs with multilevel correlations; and explore school-level predictors of ASSIST global scale scores. Study 2 further examines reliability over time and convergent validity using repeated ASSIST and Classroom Assessment Scoring System--Secondary (CLASS-S) observations in classrooms of 335 teachers in 41 middle schools. The ASSIST global measures exhibited moderate to good reliability across three same-teacher observations (ICCs ranged from .69 to .82). Associations between all ASSIST and CLASS-S scales suggested close correspondence of the measures, especially at the teacher level and school level. Together, these findings highlight the utility of the ASSIST observational measure, both as a research and practice tool, across multiple school types and classroom contexts, and with potential to inform coaching to improve teachers' classroom management.