Teachers' judgments of students' academic achievement are not only affected by the achievement themselves but also by several other characteristics such as ethnicity, gender, and minority status. In real-life classrooms, achievement and further characteristics are often confounded. We disentangled achievement, ethnicity and minority status and examined whether the achievement of ethnic minority students is judged according to the predominant expectation (expectation hypothesis) and whether teachers' judgment accuracy is influenced by students' ethnicity or their minority status (ethnicity hypothesis or minority hypothesis). We conducted 4 experimental studies with a computer simulation (the Simulated Classroom). In Studies 1 and 2 with N = 34 and N = 30 participants, we implemented Turkish (Study 1) and Asian students (Study 2) as minorities. In contrast to the expectation hypothesis, the expectations attributed to the achievement of ethnic minority students did not bias teachers' judgments. In both studies we found greater judgment accuracy for ethnic minority students, thereby probing the ethnicity hypothesis. In Study 3 with N = 48 participants, we further disentangled ethnicity and minority using German students as minority students, thus probing the minority hypothesis. Again, minority students were judged more accurately. Implementing gender (male vs. female) as the minority characteristic in Study 4, with N = 52 participants, yielded the same result: Minority students were judged more accurately, therefore supporting the minority hypothesis. Thus, classroom characteristics need to be considered in research on teachers' judgment accuracy to clarify the influence of individual student characteristics and composition effects beyond individual effects.