To not teach children about sex education is leading them to ignorance. Parents wish to teach their children themselves, that's perfectly fine. Better to tell them before their sex ed classes then. But sexual education classes allows them to ask questions they may not feel comfortable asking their parents about. And also, a lot of American schools I'm aware teach abstinence over safe sex, and this is why sexual education classes are important, because you'll find a rise in sexually transmitted infections and teen pregnancies is mostly due to the fact that sexual education is not taught at all or how it should be. Sex is a natural thing, and the more we make it seem like it isn't, your actually shaming teenagers and those in sexual education classes into thinking that they shouldn't think or have sex until they're married or their parents say it's okay. Which is ridiculous because teenagers are going to have sex and there isn't really much you can do to stop then. So the best way to take care of them as a parent should is set boundaries by teaching them about sexual education, and the best way really is through schools so that they learn everything and anything they want to know, preventing unwanted pregnancy and STI's.
The biggest problem with public sex ed (at least in my experience and the experience of people I know) is that it is taught by gym teachers and others who have no qualifications. To spare you the details, I learned some very disturbing things in sex ed that took years for me to discover were untrue.
That being said, parents do not get to decide when children are ready to learn algebra or when they are ready to read larger books. The fact of the matter is that if you send your child to a public school then the school is tasked with teaching your child EVERYTHING they need to know to be a functioning adult. If you want to control your child's education, home school them.
Some parents or guardians may find this uncomfortable, especially in certain cultures and customs. Children/teens experiments. Thats a fact of life. Furthermore, they are greatly influenced by their peers. Having a safe sound advices and information regarding what could/should happen is better than leaving them unprepared and solicited unfairly by their peers. Better know and not need it than otherwise.
I just don't see why it should be mandatory? The parents should be able to decide whether they want their kid to learn from the teacher or not. And if not the parents, then the guardian or whatever. No one should be forced to listen to the "birds and the bees" from some teacher that they may have met once at a parent-teacher conference. It just shouldn't be mandatory,
Teaching about puberty and "the ways of life" is the decision of parents, not the board of education. Parent's should decide when their kids are ready. Every kid is different, you can't just force it. Plus these teachers could not have had proper training. What if kids learn stuff they are not ready for?