As long as we determine how much of their students' success is attributable to the teacher, it seems reasonable. It seems we could make those sorts of judgments by:

• comparing the success of the students of a teacher with the success of students of other teachers having equally talented students (e.g., compare Boulanger's students' success with that of students of contemporaneous Fontainebleau teachers); or
• when successful people have typically studied with many different teachers, asking them how much of their success they attribute to the influence of their various teachers.