I have a view from both sides of this issue. I have school-age kids and my wife is a math professor (though college level, not pre-college). As a parent, I'm aware how important in life it is to have a good college on your resume. And how much more important it will be 20 years from now. I have the fortune to be able to show a STEM degree from a well respected (at leas in HN world) institution; this gave me a huge boost in my career. The odds to be admitted at such a university nowadays, if you are educated in the US, are lower and lower each day. Every single day, the US gets a little bit behind in the education race. That's why lots of upper-middle class parents are totally obsessed with their kids academic performance. Tiger-parenting and all.

Now from the other side, my wife is a model of efficiency. She receives lots of emails from students disappointed with their grades (even today she got some; we are in post-exam period). She's very calm about it. She knows she did her job well, so she simply doesn't care about whiners. She never had to deal with an escalation though (i.e. lawsuit, arbitration, etc).

I guess my point is that teachers have to simply get more thick-skinned. Just do your job, and let the whiners whine all they want. They are scared stiff themselves after all.