When my daughter was six, I averheard her friend asking if she was having sex with her boyfriend! When her friend had gone home I asked her if she knew what sex actually was and she replied, 'it's where you get naked and kiss and cuddle in bed'.
The birds and bees talk, I think should be done when they start showing that they are curious about it or, as already said; when they ask.
But I think that it's important that before they go to a mainstream school they are taught that their bodies are their own. That the private parts are to be touched by noone other than themselves and to come to you if they have any questions or concerns about the goings' on at school.
Kids are growing up way too fast these days and they are more aware than any of us can imagine. Even if they are completely innocent and showing no signs, there may be a child or two in the classroom that is more advanced or has experienced or witnessed more than they should within the home.