We have to move beyond this notion that we cannot measure what students know. We may not have the perfect instrument now, but virtually every other profession, including doctors whom we expect to make real life and death decisions, have found objective ways to sort different performers. Arguers against using student results as the basis for teacher evaluations seem to believe that education as a profession is exempt from being accountable for results.
Food for thought. In the Fort Worth Independent School District in Texas, teacher evaluations and pay bonuses are based on measures of team performance through the Texas state test. Certain bonuses are given to teachers whose whole grade level does better across all subjects. Another pot of bonuses goes to teachers of the same subjects for students who do well across grade levels. This includes art, music, and other untested teachers in the mix, recognizing that teaching is a team sport. If the evaluation system for teachers were a better reflection of the values and culture of teaching, would accountability for results be so divisive?