Here's an authoritative source: Me.
I'll author it for you right here.
You yourself, SF, have stated that all teachers who work the same hours are doing the same jobs, and should be compensated the same way. The Union should fight for this to be the case.
Lets say there's teacher M. Teacher M teaches math, has a graduate degree, but quite the private sector because he didn't like it. Gets his credential and teaches HS math ranging from algebra to calculus. He works hard for a few years, gets tenure, works hard for a few more years, his students do ok, say 90% of his students pass the courses. Then something goes wrong, he stops caring about the job. Starts to not care about the students, and his performance goes down. Now only 60% of his students pass. He blames it on kids not paying enough attention, no parental involvement, video games. He has a whole lot of studies to back up the poor performance of his students, and there is no accountability for his own lack of interest.
Now lets say there's teacher C. Teacher C teaches Chemistry. He got an undergraduate degree in education, and spent most of his free time and extra courses taking Chem classes and such. Could have probably gotten a Chemistry degree, but stuck with education as he wanted to be a teacher. Goes right out after getting his degree, and starts teaching high school. Works his guts out to build up local support for a good chemistry program. Gets students excited with fun and safe experiments, that teach them the basics they need as a foundation for any interested in chemistry or other related sciences. Develops relationships with city and business officials to help him get supplies for the classroom. Spends his whole life as a teacher in one school, getting students to pass his classes at a rate of 95%, with one of the highest percentage of people taking AP Chem, and scoring high on it as well in the entire state.
The teacher's union is dedicated to making sure Teacher M, gets the same wages and benefits as Teacher C. Granted, they may hold up Teacher C as a great example of their teachers. In fact, they'll probably use him as an example every time they try to negotiate more, or threaten a strike and show how damaging losing his teaching ability would be. Teacher M will never be brought up of course.
Of course, the state can't afford to pay all the teachers what Teacher C really deserves, because they're a public service and have only limited funds. But the union insists that Teacher M get the same pay and benefits as Teacher C.
This seems perfectly fair to the state, and it will hold off the threat of a strike, so that's the way things work.
Thus, Teacher C, who should be rewarded by vast raises throughout his entire career, has a modest set of pay raises and cuts as budgets demand from time to time. The union has hurt him despite his overperformance.
Teacher M, who should have been fired as soon as his performance dropped, is propelled forward to a long career of underperforming, but keeps getting paid the same as Teacher C, who has elevated his own mediocrity. The union has rewarded him despite his underperformance.
QED
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I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve.