6. My colleagues have given you very good, insightful answers.
Here are a couple more answers from someone with many years of experience and on the verge of retirement. This will be my final year of teaching.
Has the quality of public education really gone downhill since I was a kid, or is that a myth? No! We teach more content and more difficult content at a much earlier age now. Many kindergarteners can read --- I don't remember reading until first grade. Same with math. Algebra, geometry, etc. have all been incorporated into elementary math and have been moved up a year or two in middle school.
Have kids gotten harder to teach? Yes, there are far more behavioral problems that interfere with teaching the rest of the class. I never heard of ADD, ADHD, ODD, and myriad other disabilities until maybe the 1980s. Every other kid has an IEP or a behavior contract, or something similar. I wonder what happened to everyday "normal" kids.
Are parents working too hard to help their kids, or to notice if they're failing, let alone get involved with the PTA or pay attention to school board elections? No, not enough. The outstanding students have very involved parents. The problem kids have absentee or disinterested parents. It is impossible to get any cooperation (such as returning a phone call) from some of them. I think some of them just give up.
Are class sizes too big? YES.
Are populations more diverse? Sure, but so is the country overall.
In inflation-adjusted dollars, are we spending less per child than we were 50 years ago? No idea.
What factors are actually different now from when they were back when public education was strong (at least, it was strong at my school when I was growing up)? What are the causes of those changes, if any? There is an organized assault on teachers and public education by the republicans and their ilk.
Where is the union, what role has it played? How are union leaders selected? Our union is very involved and well-organized (in PA). There is virtually no union representation in most states though, particularly in red states. We elect our officers and we still have payroll deduction for dues. Political donations are VOLUNTARY, contrary to RW talking points.