In a valiant attempt to defuse the ideological conflicts between the reformist and traditionalist wings of the liberal education wonketariat, Matthew Yglesias argues that this disagreement is not not ideological at all. Rather, it is an artifact of past decisions about educational structure:
Take, for example, the hot issue of teacher compensation. The traditionalist view is that teachers should get paid more for having more years of experience and also for having more degrees. The reform view is that teachers should get paid more for having demonstrated efficacy in raising student test scores. This is an important debate, but I think it's really not an ideological debate at all. I think the only reason it's taken on an ideological air is that unions have a view on the matter and people do have ideological opinions about unions in general. But if we found a place where for decades teachers had been paid based on demonstrated efficacy in raising student test scores, then veteran teachers and union leaders would probably be people who liked that system and didn't want to change to a degree-based system. Because unions are controversial, this would take on a certain left-right ideological atmosphere but it's all very contingent.
This is a very interesting thesis, but ultimately I think it's wrong. There is a reason that unions kill merit pay, and it's not because they just happened to solidify in an era when merit pay was out of fashion.
To state the obvious, unions negotiate ironclad contracts to cover dozens, hundreds, or thousands of workers. Once they take effect, those contracts are rarely renegotiated, and they apply to every single worker no matter what the situation. So unions are always going to be looking for the simplest, least subjective metrics by which to measure their members. Furthermore, they will be looking for metrics which are not under the control of the other side. The school board cannot change how many years you have in service, or whether or not you have a degree. But it can change the curriculum, or the tests.