I can immediately think of four reasons why student test scores should not be counted as any direct percentage of a teacher's performance appraisal: (1) it is inherently unjust to judge person A on the basis of person B's performance; (2) any such formula is by definition an instance of the fallacy of composition; (3) it will destroy teacher morale, and therefore end up costing taxpayers more at a time when we haven't money to pay for those increases; and (4), worst of all, the backwash effect of any increased emphasis on testing will continue to degrade the educations our students are receiving.
(1) I am completely willing to be judged on my performance, but given my experience at Locke High School, where I heard of students making funny face designs out of their Scantron answer sheets on tests which held no stakes for them, I believe the potential for abuse is too great to make important decisions about people's employment and careers on such a flimsy basis.
(2) The most valuable class I had in college was Philosophy 9, Practical Logic, at UCLA. One lecture dealt with logical fallacies, and one of those included was the fallacy of composition, which assumes that the whole is equal to the sum of its parts. It isn't; the arrangement of the parts matters as well, and it is often difficult to completely identify all of the parts in any complex activity, of which teaching is one. Imagine Mrs. Lincoln being asked to give her review of Our American Cousin: we can imagine her dutifully filling out a form about the play's plot, acting, direction, and so on; there would not likely be a category about one's date's safety included on the form, but the Ford Theatre's lack of security had a decisive influence on her experience of the play ("Other than that, Mrs. Lincoln, how did you like the play?"). Similarly, there are far more aspects to successful teaching than are being discussed in any of the reductive formulas being passed around, and the last thing we need is a formula-based simplification of the teaching profession akin to the simplified reductions in curricular learning we have seen as a result of the formula-based assessments that proliferated after No Child Left Behind.
(3) I have read that, wherever such formulas have been tried around the world, they have been intensely disliked, and it's hard to see why we should now expect an exception. Teachers' morale is already at a historical low, as recorded on surveys going back to the 1980s, and as we drive young people away from entering such a badly appreciated career, the only way to attract and retain people in a job whose intrinsic satisfactions have so badly declined will be by compensating teachers with higher salaries, and our states and taxpayers do not have big pots of money lying around waiting to be drawn upon to pay those higher wages.
(4) I used to teach Advanced Placement English as well as classes preparing those students who had already failed California's High School Exit Examination multiple times so as to help them pass that test, so I am well acquainted with teaching to high stakes tests; but many subjects (art, music, vocational, and others) are not oriented towards such tests, nor should they be; and we have entirely too much testing going on already, particularly in our increasingly miserable primary schools. A predictable consequence of this appraisal proposal will be having teachers whose job security and salaries are dependent on their students' scores not put students first and do what is best for them, but instead put themselves first by turning their classrooms into appalling test prep factories. I have already noticed a change in my younger son's school, from which he graduated this week and from which his older brother graduated eight years earlier. The older teachers in my older son's school knew how to teach interesting lessons and units that did not end in some sort of standardized test, and so the best part of the school year was after such tests were finished, in May. By contrast, the younger teachers and new leaders in my younger son's school seem to have no idea what to do once the testing is finished; the last five weeks of the school year are a round of parties and games and are devoid of learning, a complete ripoff of the taxpayers, and this apparently has met with administrative approval.
I know some will think I am switching sides here, but I believe reform has gone badly off the rails in the last few years, and I urge the teachers in Los Angeles and similar communities to reject this bad idea being promoted by well intentioned people.