I blame Horace Mann.

Why We Lost The Republic
Tim Daughtry

Now, after years of patient effort, the teachers' unions have turned America's schools into a wholly owned subsidiary of the political left. Conservatives have complained when reports surfaced about students being taught to sing hymns of praise to Obama, or when conservative students were harassed in class, or when examples of blatant liberal bias in textbooks came to light, but somehow we allowed ourselves to write off public schools as a lost cause. Homeschooling and private schools gave some relief, but the idea of getting public schools to transmit a love of liberty and appreciation for free enterprise seemed hopeless.
...
As long as the left has controlled the schools, time has always been on their side. We lost the Republic in the classroom long before we lost it in the voting booth.

It's unreasonable to expect teachers in government schools to argue against the existence of their industry. "Public education" (i.e., tax subsidization of  schooling and policies that restrict those subsidies to schools operated by government employees)  originated in mid-seventeenth century Protestant evangelism and early nineteenth century anti-Catholic bigotry. We live with the consequences of a centuries-old mistake.

Update

The terrifying thing about modern dictatorships is that they are something entirely unprecedented. Their end cannot be foreseen. In the past, every tyranny was sooner or later overthrown, or at least resisted because of "human nature," which as a matter of course desired liberty. But we cannot be at all certain that human nature is constant. It may be just as possible to produce a breed of men who do not wish for liberty as to produce a breed of hornless cows. The Inquisition failed, but then the Inquisition had not the resources of the modern state. The radio, press censorship, standardized education and the secret police have alterted everything. Mass suggestion is a science of the last twenty years, and we do not know how successful it will be.

George Orwell, Review of Russia under Soviet Rule by N. de Basily (George Orwell, Essays, Knopf, 2002).