I guess I just never noticed. Or maybe it didn't happen and conservatives are just making it up to justify why people don't want to vote for them.
I definitely noticed it in my time in public schools.
One of our civics projects was even to do mock business scenarios for union & non-union environment while doing that part of the course.
They had one student play the company owner and was instructed "You can made whatever rules you want", and then the other students were supposed to be the employees. Of course, it was setup intentionally to give the impression of "look how much better it is for the employees when they're unionized!"
...now, I'm not making a claim that it's some sort of "master plan" by democrats to keep their power by indoctrinating children...I know some other conservatives have that theory, but I don't.
I simply just see it as I see any other situation where adults are teaching youth... Adults are never going to be able to completely detach their personal feelings from the subject matter they're teaching when it comes to politics. We'd all be guilty of that...if I taught a history course, my libertarian opinions would show through regardless of how objective I tried to be about the subject matter.
It just so happens, that a large number of public teachers are left-leaning and pro-union (considering that most teachers are in situations where union membership is mandatory for their employment)...and most people who are pro-union tend to vote democrat.
...so that's what I'm referring to when I say that public education contributes to the next generation of democratic voters.
I don't think it's any coincidence that almost 70% of voters 18-21 vote Democrat. If you talk to anyone who's in that age range, they're not particularly informed on many of the issues (not knocking them, it's just that people in that age range typically have other interests), obviously that skewing isn't coming from their parents, if it was, we'd see that same 70/30 split among older age groups, but we don't see that. So if it's not parents that are influencing this, what other adults, who've had a large part in their lives, would have encouraged this? Doesn't leave too many other people other than clergy (which is highly unlikely) and teachers (far more likely).
...but like I said, I don't think it's some Democrat conspiracy to indoctrinate youth, I just think it's the fact that anyone who teaches is going to allow their personal opinions dictate how/what they say to a certain degree...and most public school teachers happen to be (D) voters.