For all those saying 'shit, let them find it out for themselves/their friends/internet tell them, /when they ask, explain...'
NOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO
This is how you get people like my friend who thought that girls pissed through their vagina, didn't know what a period was, and people thinking that doing it standing up stops you getting pregnant, because, y'know, gravity, man. You cannot rely on a 10 year old to question their own or their friends' knowledge of anything, or to cover all the bases; the internet cannot tell you how to make love, only how to make a woman airtight.
It is also how you get teenagers becoming massively insecure about being virgins, when all their friends are talking about having done it already - regardless of whether or not they have. Time has shown that the more and the earlier children learn about sex, the less likely they are to do it before they are ready (which of course varies from person to person.) My mum sat me down when I was about 7, and I didn't do anything potentially pregnancy-worthy til ten years later. School was bullshit.
To all the boys on this thread saying 'let them ask/find out themselves:' like to see you explain periods. A lot. That would be hilarious.