I think one has to look at the history of teacher's unions in the US. For example, unions were formed because women could be fired as soon as they got pregnant. The role of unions is to protect their members from arbitrary firing. In the current era, you have issues such as teaching evolution, etc. Unions can protect their members against arbitrary firing--what schools need to do is document truly atrocious behavior and then fire teachers that engage in it. However, one should consider that top scorers in PISA, such as Finland, have teacher unions--they also have well educated, well paid teachers, with a lot of autonomy.