According to dimensional comparison theory (DCT), students evaluate their ability in one domain (e.g., math) by comparing their achievement in that domain with their achievement in other domains (e.g., English). Primarily in research on students' academic self-concept, these comparison processes have been found to lead to positive associations within subjects (e.g., the better the student's achievement in math, the higher that student's math self-concept) but negative associations between subjects (e.g., better skills in math than in English lead to a relatively lower self-concept in English than in math; Möller & Marsh, 2013). However, less is known about dimensional comparison effects in evaluations of others, for example, students' ratings of their teachers. In the present study, we used data from a large-scale assessment of teaching quality in Germany (N = 6,479 students from 401 classes in Grades 5 to 10) and examined the associations between students' grades and their teaching-quality ratings in mathematics and German language classes. In line with DCT, the results revealed positive within-subject associations and negative between-subjects associations: Most importantly, dimensional comparison effects were also found at the classroom level and remained stable even after achievement test scores were controlled for.