To a certain extent, good manners consists of overcoming our natural instincts. We shouldn't grab food from other people, we shouldn't hit people if they anger us, and we shouldn't mock anyone for something they can't help. That's just basic decency. To the extent that schools help civilize students these ways, they are a positive influence.
Sex education teaches basic precautions. No one would make a serious argument that letting that particular instinct run wild would be good for society. Sexually transmitted diseases and early pregnancies are best avoided by everyone.
But much of what the schools teach in this politically correct era seems to have gone beyond basic manners, and is now about propaganda.
My son went for one year to a private school where they told the students that sexual harassment included any sort of suggestive comments, which could even mean complimenting someone on their appearance. ("Hi Susie, you look nice today" -- arrest that man!)
They even said that staring at someone too long could constitute harassment. (It seems to me I spent my high school years staring longingly at various girls but doing nothing about it; I had no idea at the time what a criminal I was.)
One day this school had a guest speaker who informed the students that if they'd ever had sex while drunk, that meant they'd been raped. (I guess I've been raped, though it's probably too late to report it to the authorities.)
Another thing the schools teach these days is tolerance for gays. This is good; any fair-minded person would agree that no one should be blamed for what they have no choice over. But at the same time, it's unnatural. Most guys are instinctively repulsed by the thought of two men having sex. So once again, students are being indoctrinated against their own instincts.
The most complete indoctrination goes on in the name of race relations. Students are taught that noticing any differences -- such as in crime rates, or test scores -- between the races is absolutely taboo. Yet noticing such differences, and thinking empirically, is what people do naturally.
White students are taught that they must bend over backwards so as not to offend black students (brown students don't get nearly the same deference). To that end we have Martin Luther King Day, black history month, and Kwanzaa (a holiday which was never really celebrated in Africa).
But this goes against natural instincts as well. Evolutionarily, we're all programmed to stick up for people who look like us -- our extended family. This goes for black people and brown people as well as white people. But white people are encouraged to be ashamed of their imperialistic heritage, and to kowtow to the other races.
But that's what education is all about these days. You're taught to ignore your natural instincts. And if you can't quite manage to completely squelch them, you're taught to at least feel guilty about them.
Much of liberalism is in fact a continuation of the same: who you gonna believe, me or your lyin' eyes?