The problem isn't necessarily the ability to pass a test. It is their ability to teach.
It is not unquantifiable. It is not a mystery. Believe me, all of the parents talk and word of the best teachers get around and believe me it is hard to get them with these good teachers.
The problem is not that we do not test the teachers or we test too much. The problem is that we have not privatized the system. We are trying to implement processes that would emulate the free market. Unfortunately, these imitations fall far short of the real thing.