The issue isn't that teachers don't want to be assessed (at least not for most of them). It is that they don't want to be assessed using shitty metrics.
Things are getting somewhat better because states are moving towards growth models that look at a student's net improvement rather than just measuring them to a flat standard. So you are expected to make student improve, but not expected to make them all model students regardless of their starting point.
So problem solved, right? Let's have the teacher's union promote the use of this metric!
The other issue is with standardized testing. The only thing it really measures is how good students are at taking standardized tests. This has perhaps some connection to "intelligence" or "knowledge" or whatever the hell it is we're trying to produce through the education system, but the thing is we don't really know what it is we're trying to produce, much less measure.
Everyone like to parrot this, but the fact is basically every university looks at SAT scores as a major factor (not only, but still major) in deciding which student to accept. Unless you are going to claim that all universities are crazy, it IS a strong indicator that YES, standardized tests IS A USEFUL MEASURE of a student's ability (not to claim it is the ONLY measure, but a USEFUL measure nonetheless).
Furthermore, even if you insist that whatever a standardize test measure is still useless, you cannot dispute the fact that a good score WILL help the student in getting into the university he/she wants to, which itself is of great use.
Seriously, intelligence, knowledge and education are not well defined concepts, and attempts to quantify them as a single number are misguided. There's been some effort in these areas recently, but it is rather backwards that we've started out by attaching nationwide policy and billions of dollars to these things before we even have any idea of what exactly they measure, how reliable they are, and what the issues are with them. Trying to base your entire assessment of performance on a concept that is not well defined, much less measured, is a good way to irritate your employees, and teachers are right to bitch about this being a stupid form of assessment.
So because we don't fully understand a thing, we should even TRY to measure it?
Well, politics is also a very ill defined concept, yet it doesn't stop people quantifying them as a single number (number of votes) and hold elections to decide who gets to be the POTUS. By your logic, we should just do away with election and have a tenure for POTUS.
You wouldn't blame an IT person who complains that their only metric is number of tickets closed, when that has no bearing on whether the problem was actually solved correctly, thoroughly, or at all.
That IT person can complain all he wants, and is free to leave for another job. Yet you wouldn't say that because a shitty metric
is used, that IT person should have a lifelong tenure at this job, would you?
This isn't a question of whether teachers should be assessed. It is a question of how they should be assessed, and measuring all students from all regions against an arbitrary, fixed standard is a piss-poor way to do it that has little bearing on whether a teacher is good or not.
Yet in the discussion here, you only see the argument for NOT doing assessment at all because the metric is shitty, but you rarely see any counter-proposal for a non-shitty metric. Just coincidence? I think not.