My mother enjoyed telling the story of a little boy who proposed to me in nursery school (and threw water on my dress when I turned him down. The nerve!) Apparently his parents were very happily married and he wanted to be just like them.
Children don't live in a vacuum. They watch tv, they observe the adults around them, they learn from each other. They may get wrong ideas, and that's where education comes in. And for those saying parents should teach their kids the facts of life, they are often waaaay behind the curve, if they bring it up at all. Mine certainly didn't.
Little Charlie's parents didn't realize their son wanted to get married at the age of four. Our teacher had to tell them. No harm was done (except for my dress!) but it makes the point that teachers see how children act with their peers, which may be quite different from how they act at home. Teachers can provide useful guidance at important moments in a child's development.
In Charlie's case, that amounted to "Wait till you're 18 to propose, and stop throwing water on girls." For other kids, at other ages, it can literally be a life-saver. Children are not the property of their parents; they are individuals who deserve to know the facts of life in order to make their own decisions.