Purpose: Prior work has drawn consistent conclusions about systematic racial disparities in the allocation of high-quality teachers in US public schools, such as classrooms with more students of color having teachers with fewer credentials and less experience. However, these fixed characteristics of teachers are only proxies for the quality of teaching, which may vary within teacher by the different classrooms they teach. Research Methods/Approach: Using data from the Measures of Effective Teaching (MET) project, we consider various sources of within-teacher, across-classroom variation of teaching effectiveness using a teacher fixed-effects modeling approach and highlight those that may, on average, disadvantage youth of color. Findings: We find that (1) about half of the variation in classroom teaching efficacy is within teachers, (2) classrooms taught by the same teacher with higher percentages of Black and Latinx students receive lower quality of teaching, and (3) these patterns are consistent across teacher racial/ethnic groups. A number of plausible explanations of this association are considered, including rater biases on observational measures and differences in teaching practices; we also consider how these variations differ by teacher race. Implications: Our findings highlight the importance of teachers' in-classroom practices, not simply their credentials, in educational research, policy, and practice targeted at reducing racial inequality.