I'm not fan of corrupt unions (and trust me, teacher unions are corrupt). But this is not a union problem at all. It is a central administrator problem.
A number of years ago, a major public school district close to where I live dropped their civics requirement. Imagine that, learning about how the US government works was no longer mandatory! Instead, students could choose from a list of options: US civics, or world cultures, or black studies, or Latino studies (I'm sure you can see what's happening here).
This was entirely the history supervisor's decision. Almost every history teacher was outraged, and many of them filed complaints. It didn't matter. The supervisor stuck with his decision. The School Board (and the voters) did nothing.
Excuse me .. the Administrators are MEMBERS OF THE UNIONS.
The NEA has been very aggressive in putting through all those class changes you were talking about. This is why the youth of today do not know American History; unless they are home schooled, or they go to a Christian or Charter school.
The unions are aggressively supporting Common Core, to further dumb-down our children.
And .. ALL TEACHERS (whether functioning administratively or not), are members of Teacher’s unions.
Why do you think that no teacher ever gets fired ..?? On occasion, there is an administrator who will do the right thing; but it’s more likely the union will pressure the school to keep a terrible teacher.