Product evaluation takes decades. Only years after graduation can one evaluate how good a job the school did. Schools that consistently produce a bad product often get the most money. Worst of all, the people who run the public schools have little real control over policy or personnel. That is in the hands of the teachers unions who run the school for the benefit of their members.
Teaching is only one of many factors that affects student performance.
Outstanding teachers may push mediocre students to undreamed of heights, but they control neither their raw material nor the educational environment of their school. A superb teacher stuck with craven administrators, a lousy curriculum, irresponsible colleagues, and impulsive students may find his best efforts washed away on a tide of disorderly conduct. At the other extreme, excellent students often perform superbly in spite of rotten conditions. If performance means scores on standardized tests, as is usually the case, rational teachers will do two things. They will teach to the test and they will lobby to ensure that performance goals are low.