abstract
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Using TIMSS 2019 mathematics data, we investigated the variance in achievement within U.S. fourth and eighth-grade classrooms. Approximately 23% of students in a typical grade four classroom are expected to score at or below the low benchmark whereas 14% meet or exceed the advanced benchmark; these numbers are 35% and 14% for grade eight classrooms. Approximately 69% of grade four and 35% of grade eight classrooms included students across all four TIMSS benchmarks. About 68% and 37% of the variance lies within classrooms. Grouping students primarily based on age appears to yield classrooms with vast academic variability.