It isn't about "hating" unions, or otherwise, but about recognizing the simple fact that unions aren't magical unicorns, they are interest organisations and the teachers' unions represent the interests of the teachers. Now, those interests are often aligned with the interests of the students, but students' interests do not flow from teachers' interests.
To get back on topic, teachers are opposed to objective measures because if you can objectively tell if a teacher is bad, the union will lose influence over pay and employment security, which is where their power is anchored. Thus, they will of course promote studies that show the failures of such objective measures and try to discredit those that show the opposite. Just like any other interest organisation.
That doesn't mean that I have a problem with the existence of interest organisations, but let's not pretend they are something they're not.
Of course. But the whole point is that we shouldn't treat a teachers' union as an authority on whether it does (or can) exist. And no, it's by no means an established fact that it can't.
To get back on topic, teachers are opposed to objective measures because if you can objectively tell if a teacher is bad, the union will lose influence over pay and employment security, which is where their power is anchored. Thus, they will of course promote studies that show the failures of such objective measures and try to discredit those that show the opposite. Just like any other interest organisation.
That doesn't mean that I have a problem with the existence of interest organisations, but let's not pretend they are something they're not.