You're too focused on bureaucratic nonsense (unions vs. administration vs. state vs. local vs. federal government.)
In fact, it seems like you're just taking it as a given that unions are the problem and everything will be made better if you take away the teachers' first amendment rights to assembly and petition for redress. Which has nothing to do with teaching students, which is the problem at hand.
But that is the point! If the union makes it impossible to fire a bad teacher, you must try to limit the damage that teacher can do, hence the set curriculum. If the union makes it impossible to single out bad teachers for this kind of prescription, then you must do it to ALL teachers.
Get the union out of the picture and the situation becomes self-correcting.