In many studies of class size effects, teacher characteristics are missing, even though many argue it is not class size that is important but teacher quality. In the present study teachers' effectiveness on the learning progress was assessed while teaching a unit with predefined learning objectives. To measure adaptive teaching competency a multi-method approach was employed (e.g., vignette and video test). There were 49 teachers and 898 students. Smaller classes led to higher academic learning progresses, better knowledge of students, and better classroom processes. Adaptive teacher competency remained relevant in smaller classes, that is, class size and teacher quality were independently important. (Contains 5 tables and 4 figures.)