The interference being run has to do with (1) merit pay being a quite deliberate attempt to break current bargaining methods so that overall teacher pay can be reduced afterwards, and (2) the metrics for “merit” being extremely difficult to do in any reasonable way, since every student is different and (for instance) standardized test scores are a pretty terrible proxy for learning.

(to further explain (1), in particular cash-scrapped governments and school districts would be all to happy to get to fire a bunch of older teachers who, because teaching was previously set up as a career, make more money than new recruits. This is precisely the kind of logic that meat packing plants or furniture factories also apply in firing veteran employees and replacing them with desperate undocumented immigrants or other similarly cheap new workers, and countering it, so that pay is predictable and stable for people being recruited to do extensive training and then work in what is frankly a quite low paid (compared to the responsibility and workload bestowed) career, is pretty much the entire point of labor organizing: it removes the tremendous power imbalance that otherwise exists between employee and employer.

I absolutely agree with you that our current methods of selecting, training, and critiquing teachers are deeply flawed. But they’re flawed for a whole host of reasons, and the teachers’ unions that fought against the often worse problems of the pre-union past are not by a long shot the biggest problem facing the education system.