How can we measure teaching excellence?
With extreme difficulty.
Is a teacher who enables you to pass the next lot of exams better than one who inspires you to love the subject and study it?
Leaving aside the consideration that exam results are are simply a measure of how well you can do exams - nothing more than that - consider the following. Two teachers: one with a B-grade student and the other a D-grade student. The first student achieves an A-grade and the second a B. Which teacher is better?
Value-added is not a simple measure. I know of at least one school where the level of students at intake is lowered (bad assessment or bad intention?). As a consequence the school's apparent value-added is inflated. Or consider two pupils who enter at the same level. One has a home environment which supports and encourages academic performance. The other comes from a home where such support is lacking. Two teachers work with one each and get them to finish with the same grade. Are the teachers equally good? All that you know is the pupils' level at entry and leaving. Details of the home environment are not necessarily known to the school.
Furthermore, teaching is a team effort. A good mathematics teacher could help you do better in the sciences even though your science teachers are pretty average.
Feedback from pupils can be useful but less so for very young pupils (they can "love" their teachers for all sorts of reasons), intermediate level pupils (they may be biased by the amount of homework, whether or not they are good at the subject). In retrospect, I can see that one of my best school teachers - in terms of what I learned from him - was an unpopular disciplinarian. But he knew his subject and could communicate that information to a bunch of youngsters. Had I been asked for feedback when I was at school, I would have been highly critical.