The Los Angeles Times will publish evaluations of over 6,000 elementary teachers on its website (story here), which reflect their students' improvement on test scores. The union, predictably, rages against the injustice of it all, and attacks the administration, the evaluations' statistical process, and the media. What the union does not do is suggest how to properly evaluate teachers.
Education is difficult to quantify, especially with so little agreement on educational goals. Teachers have a justifiable fear of both individual principals' reviews, which can reflect political and personal issues, and calculated scores which cannot capture a teacher's full impact. What does? Probably some combination, much like other professions. I get scores and commentary on my reviews - we don't calculate our numbers, but some companies do.
I can't put into words how much I respect teachers (and am frustrated that I lack the skills to be one), but we need a way to eliminate the bad, reward the good, and spread best practices to help the rest. Educators should lead the charge with effective accountability that parents and taxpayers can understand. The current system obstructs this kind of effort, but if teachers don't find a solution, they'll have to live with the system's.