My girls did this when they were Juniors (they're cadettes now). We had a nurse come in to discuss the technical stuff, and we had a couple of teenaged girls come in to talk to them. One of the biggest eye openers was the variety of ages that girls start puberty now adays. At that time, most of the girls were in late 5th grade. Only one girl had started her period, but she had begun at age 9.
All the adults who were usually with the girls talked about our experiences too, which really helped. The girls were able to see that it's not all just gross stuff, and that if we could talk about it, that it wasn't going to be the end of the world for them.