Nobody is suggesting bringing sex into a child's world. As Anya reminded us, this is about sex and relationship education. Unfortunately, there are children as young as four or five who are sexually abused. I expect you told your own children not to accept sweets or lifts in cars from strangers, but didn't go into details of why they shouldn't. This is just the updated version.
You seem to assume that all parents are loving parents, who are happy to talk to their children about relationships and sex. Sadly, they aren't. The reason I mentioned the age which I started my own periods was because they had never been discussed and, being so young, it affected me for years. I would have been so much less anxious, if my mother or a teacher had discussed them and explained that there was nothing wrong with me. My boobs also started growing about the same time and I remember boys in my class wanting to touch them. I had no idea why, but I was uncomfortable about it and didn't know how to react. I wish I'd had the confidence to tell people it was my body and I had the right to say 'no'.
Secondary school is far too late to start sex and relationship education.