Professional knowledge of teachers is a major prerequisite for teaching, process-related aspects of teaching quality, and students’ development. Concerning pedagogical-psychological knowledge (PPK) and its role for students’ cognitive development, there is some empirical evidence for this assumption. An open question is, however, whether this is also true for students’ development of situational interest and which aspects of teaching quality take over the role of a mediator. The present paper focuses on this question. The sample consists of 34 German teachers at higher-track lower secondary schools and their 973 students in physics courses. Teachers’ PPK was measured with a paper-pencil test, classroom management and managing heterogeneity, both representing aspects of teaching quality, were assessed using video analysis of course activities, and students’ situational interest was measured using the FAM questionnaire. Results of mediation analyses indicate an indirect effect of PPK on situational interest, mediated by the quality of classroom management. This finding emphasizes the major role of teachers’ PPK as well as teachers’ classroom management for developing students’ subject-related interest. As a further result, video analyses indicate that there are almost no teacher activities focusing on managing student heterogeneity. For ruling out that this floor effect is due to sample selectivity, an amendment study was conducted: In a questionnaire, 177 college students were asked about their memories of their high-school teachers’ activities concerning managing student heterogeneity. Results indicate a floor effect as well. In conclusion, both studies demonstrate that managing students’ heterogeneity does not play a major role in physics courses at German higher-track lower secondary schools.