I will state that Teach are worth what they get paid and many are not getting paid enough to deal with all the crap that goes on in schools these day.
However, unions have no value in the educational system, Teacher are all highly educated and went to a four plus years of school like most everyone today and do not need someone negotiating on their behalf. The good teacher get less than they deserve so the lesser teacher can keep their jobs. If I was a good teacher I would demand not to be in the union and negotiate my own Pay and such so I can get more for doing a better job. But teacher are required to join the union they have no choose so they all get the lowest common benefit.
At one time Teacher got way less than most people and now they have better benefit than most everyone else. This is where the issue lies, we all see them getting great benefits, our taxes going up and our kid get less of an education because the majority of the bad ones are being protected by a union contract. In the end the kids are not getting the best since we protect the worse educators.
If the student in the US out performed the rest of the world and we all were making lots of money because of it, no one would be complaining about giving the better teacher more money.
As person who grew in a Union Family I will tell you the people who run the union only care about pay increase for everyone since their salary is paid by union dues which is usually a % of your pay. So a pay cut to the teachers is a pay cut to the union leaders. I never understood why my parents paid % of their income to people who could careless about them personally.