In a talk by Kevin Williamson of Nation Review (I think it's this one) he made the comment that public education has the government as it's customer. That is, the public education system seeks to create citizens that can serve the interests of the State. This make a lot of sense to me.
In the past, I blogged about how my daughter's horrid public high school. It was four years of political indoctrination, primarily focusing on social justice and racialism. At the time, I was trying to fathom just how this was going to give my girl the knowledge she needed to function in the real world. Instead, I should have been looking at it from the point of view of the teachers, their union and the government with whom they had a symbiotic relationship. My daughter was a future vote to be shaped. That was her role as the widget the school-factory produced.
That my daughter knew practically nothing about American or world history didn't matter at all. Nor did it matter that she never read any of the classics of Western literature. All that mattered was that she had the racialist foundations drilled into her head which would cause her to lean left politically. Her votes directly affected the money flowing into the public education industry. Her votes were the return on their investment.
Now that I've got this model in my head, it all makes perfect sense. It's not even part of a huge conspiracy. Everyone involved is acting in their best interests as you'd expect. It's the natural consequence of the growth of government. As government grows, more and more aspects of our lives become political. Money and power flows more and more from the government and less and less from private organizations. The public education industry is simply doing what any business would do - serving its paying customers.