Counseling instructors using evaluations made by their students has shown to be a fruitful approach to enhancing teaching quality. However, prior experimental studies are questionable in terms of external validity. Therefore, we conducted a non-experimental intervention study in which all of the courses offered by a specific department at a German university were evaluated twice with a standardized student evaluation questionnaire (HILVE-II; overall 44 instructors, 140 courses, and 2,546 student evaluations). Additionally, twelve full time instructors received counseling after the first measurement point. Long-term effects over a period of 2 years and transfer effects to other courses were analyzed using multi-level analyses with three levels. Possible influences by bias and unfairness variables were controlled for. Our results indicate a moderate to large effect of counseling on teaching quality. In conclusion, if students' evaluations are accompanied by counseling based on the evaluation results, they present a useful method to assure and increase teaching quality in higher education.