Multitiered systems for supporting students' behavior in schools rely on teachers using generally effective classroom management strategies, such as Tier 1 practices. However, few tools exist to easily assess classwide Tier 1 practices. In this brief report, we describe one potential tool, the Classroom Atmosphere Rating Scale--Brief (CARS-B) and assess the properties of the tool using item response theory and multilevel models. We found that most items on the CARS-B had acceptable discriminations and difficulty estimates. Teachers' CARS-B latent scores were positively associated with student group on-task. We discuss potential changes to the CARS-B and its use in research and practice.