Please have the convo way before highschool.
For us it wasn't one big talk but a lots of little ones over many years. I answer any questions they have, when they have them. I find it helps to talk about while your doing something else eg craft, driving etc.... Most of ours have been in the car on the way home from school.
I have brought it at times just to gauge what they know and the crazy things they have heard from the school yard.