I am assuming she is about 4th grade? If so, in the 4th grade is when our school nyrse comes in for a talk with the girls because there are so many moms who just shudder at the thought of talking and do nothing and the poor girls have no info.
We started communicating WAY early on when daughter was a toddler and talked about everything. She knew what to expect and she also knew that IF her period started at school that she could trust that the school nurse would be more than willing to help her.
My daughter started at 12, at school, and Thank God, she was prepared. She jsut went to the nurse, got things under control ,called me, and there was no embarrassment or horror.
I got a booklet and info free from Playtex tampons and we used that info along with a lot of open communication with me listening and answering questions.
If you are not comfortable talking to her (no worries... my mom was a huge prude and I learned everything from my BFF and her mom) talk to her when you are driving, etc so you are not face to face.
I do believe you would prefer your daughter to come to you with questions and info vs her finding out info that is not all correct from friends. Thank God, I had a very loving BFF mom.
Good luck!