I'd say a below-average teacher is more likely to "succeed" in a good school district, one that is in a better community with more parental involvement, than good teacher is in a bad school district. We've had this discussion before. Privatizing education puts certain children at even more of a disadvantage through no fault of their own. I think it comes down to what you value, which in the conservative view is reward for an individual's ability to succeed in a capitalist system - the ability to play "the game", if you will. I value capitalism too, but realize that educating future generations is more important to the success of our nation than adhering to individual rewards for individual achievements.
If you really value better teachers, then make the job attractive enough so that more capable people will choose a career in education over one in private industry, where they can obviously make more money if they have the "tools" to do so. Of course, you don't value better teachers - you just don't like unions.