I went to my city's public high school that was close to 1/3 white, 1/3 black, and 1/3 Hispanic, with a peppering of Asians. My friends and I were smarter than all of our teachers save four. These four teachers were wonderful foundational influences for us, and they thought for themselves and expected us to do the same. Most of the other teachers were coaches or mothers who needed something to do that would fit into their schedules. My friends and I talked a lot about the problems with our schooling, but we didn't notice much centralized indoctrination. By that I mean there didn't seem to be some kind of single influence for all the weird ways they fucked up people's individuality and free thought. It was a combination of state programs, the teachers themselves, the school board, and the administrators. I know that sounds like central authority, but they all had different motives and goals.
The state (Georgia, USA) set a bar for everyone to achieve, but because everyone was meant to achieve it, it was extremely low. My friends and I were in an intellectual minority, so we didn't receive very much help in the way of achieving what we ought to. It was the case for all of our classes involving state-administered curriculum that everything was painfully simplified and easy. To the point where it almost wasn't even worth learning because it was either common sense or could be covered better by reading a book over the span of a week or two. These were the teachers who either didn't know enough or care enough about their subjects. The students at the bottom still couldn't handle it because nothing is ever expected of them, and the students at the top could have been doing hundreds of more productive things. It was a colossal waste of time.
The teachers for these state classes were largely uninspired. Many of them had their own personal views that they tried to embed into our impressionable minds (it's a great opportunity to spread your beliefs, I don't really blame them). Many of the male teachers would assert Christianity to the best of their ability without going outside of what was legally allowed. We had one female teacher give out dating advice to the girls based on her two divorces. Another one tried to convince a friend of mine that her shitty state-regulated economy class was more important than our college-level physics class. He had owned stocks for 6 years at that point and is now in school for engineering. There is a tremendous level of vindictive self-righteousness because many teachers are subconsciously aware of just how worthless they are compared to their coworkers. And they tend to take it out on us students by assigning hours of useless busywork to do at home. Economics is certainly important, but this teacher was completely incorrect about a lot of the things she taught. The best high school teachers I had taught AP physics, calculus, and history. For those who don't know, AP classes have their curriculum made entirely by the College Board, who also makes the SAT. It's a nonprofit organization owned by Jews, and a thousand times better than the state.
The school board honestly doesn't do a whole lot that affects the classes themselves, it mostly just fucks things up logistically whenever things were beginning to run smoothly.
The administration at the high school I went to changed a bit, but it was fairly normal for the Bible Belt. This was probably the worst in terms of restricting individuality. They are the ones who decided that school would take a tremendously large portion of the day. The dress code didn't allow for piercings (excepting ears), unnaturally colored hair (I don't like Tumblr either but humans have the right to do harmless artsy shit to their bodies), and skirts/shorts above fingertip length. Two of my younger siblings and one of my neighbors attend this school now, and they showed me the new dress code. Skirts and shorts must be knee-length, leggings are not allowed unless under the aforementioned garments, no head scarfs are allowed, and nobody can have "excessive" hairstyles. I understand the concern for teaching kids to dress well for their work environment, but most work environments aren't this strict. There is very little expression of individuality allowed.
A combination of these groups and their assertions and deficits, in addition to the large swaths of ignorant morons in the student body, makes public school what it is. It's not really a conspiracy as far as I'm aware because nobody is hiding their motives. My school's architectural layout was exactly like a prison because it's easier for teachers and police officers to watch as many kids as possible. It is what it is, and a kid who is smart and adaptable enough can make do with it. Average kids will be raised for average lives, and idiots can't be helped much anyways.