The Classroom Assessment Scoring System (CLASS) is a widely administered measure of classroom quality that assesses teacher-child interactions in the domains of Emotional Support, Classroom Organization, and Instructional Support. We use data from an evaluation of state-funded prekindergarten provided to 684 children from families with low incomes (M[subscript age] = 57.56 months, 48% female) to examine the extent to which CLASS scores vary over the course of an observational period within a single day and investigate whether this variability is related to children's school readiness at the end of the preschool year, holding constant two additional measures of quality. Teacher-child interactions in all three domains were moderately stable. Mean Classroom Organization was positively related to math, and variability in Classroom Organization was negatively related to literacy. Mean Instructional Support was negatively associated with language. Findings have implications for programs that utilize the CLASS to make high-stakes decision and inform professional development.