When I was in high school (late 70s/early 80s), it seemed the standards were looser. They had done away with the strict dress codes from years earlier. Some parents might have complained, but they were outnumbered by parents who didn't want to have a strict dress code. That's why school board elections can be so contentious, at least they are around here.
Sometimes I wonder if there are those who still carry some old resentments about their own school days, and end up being the loudest ones who complain to the school in their adult years. They might see it as a vicarious form of "payback."
Interesting theory. I was a primary school teacher, and didn't really notice that. It was more that some parents thought their kid was the most important in the class. Obviously, they are the most important
to them. But a surprisingly large number of parents would readily throw the rest of a class under a bus and accept special treatment for their own kid, without reflecting on the larger picture at all. So I'd see myopic parents as the biggest issue.
But I had a friend who was a high school teacher, and his perspective was very much along the lines of what you're suggesting. He found the hardest parents...and by extension kids...to deal with were parents who had struggled or hated school, and then been successful in their later life. Tradesmen running their own businesses was the example he commonly used. Men in particular who had not relied on school for their success, but had instead worked their way up through an industry and ended up owning a business, and employing people.
Just different viewpoints, so I can't validate it objectively, but as a subjective story I thought it might be of interest.