I think kids should just grow up with knowing. I don't think there is a time to sit kids down and educate them as a parent. Just explain as you go along how things work in an age appropriate way, certainly that was what happened in our house, I can't remember ever not knowing.
with regards to schools, I think understanding that people fall in love in many was and there are many styples of relationship ought to occur from when kids first go to school, but the sexual mechanics of the matter should be deal with around 11 years old, probably the first year of senior school and that needs to be very blunt, very detailed, very health and personal wellbeing orientated, so kids understand simply and clearly.