Back in middle school, a lot of people knew about sex, but they didn't really know about sex. It's important to not just learn from the media and smut, but actually learn from educational resources.
If you didn't understand what I'm trying to say, basically tldr;
Children should not be learning about sex from smut, but from education/resources that will actually explain it properly.
Children like to talk a lot. If you teach one child about sex, it might spread around the entire school, along with any misconceptions the child who spread it might have. Education is important, but often people are miseducated. Also, most parents don't want their children to be exposed to sex at a young age, so its best to respect other people's decisions.