High school course grades are a primary source of information about students’ academic readiness, yet they are often viewed as inconsistent measures of student achievement—influenced by idiosyncratic practices across schools and teachers, systematic differences in course content and structure, compositional effects of peers, and student demographics. Prior research has not quantified the extent of this variation relative to the influence of students’ academic skills and effort, or examined multiple sources of influence simultaneously. This study employed cross-classified random-effects models with a dataset of 2.1 million grade records from 125,223 students and 11,000 teachers at 118 schools to identify sources of variation in students’ grades. Grades varied based on which teacher students had for a given class and the conditions of the class (course subject, classroom peer achievement, time of day, class size and term). There were systematic differences in course grades by race and gender, even among students taking the same classes under the same conditions with the same test scores and attendance in the course. However, students’ effort and skills, measured through attendance and test scores, dwarfed other sources of variation. Within- and between-student variation in attendance across classes also explained a substantial portion of variation by teacher, school and course conditions. Rather than finding large unexplained differences in grades based on which school a student attended, or which teacher they had, we found observable factors systematically explained differences in the grades that students received, particularly in students’ aggregate grade point averages.
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