If you are right, (and I think you are) all of that is the parent's job. So why are we inviting the schools in to it?
Because parents aren't/weren't. If every parent taught their child about how/why their body is changing, what sex is, how to not get pregnant/STDs, what is consent, etc, sex ed wouldn't be necessary. But they don't, and this stuff is absolutely vital information for every person to know.
But a parent teaching (or school for that matter) abstinence, if followed, will prevent all that. Why is a 'don't do it' program useless?
Because it doesn't work. Studies have shown that teens who are taught "abstinance only" have almost as much sex as teens who are taught about birth control, and are more likely to get pregnant and STDs, because they're not using protection properly.