Teachers' union power varies a ton state-by-state. In mine, both our regional union and the NEA either can't, or choose not to, do anything, really, including in cases of blatant contract violations. I don't think they're allowed to negotiate those contracts in the first place, either.
I'm confused by this. There are many notable large workgroups in the US with powerful unions. Police, for example. Pilots, Teachers, etc.