Leave them in the bathroom and walk away. She is in denial and I don't blame her. If she knew what would happen for the next 30 or so years as it relates to her body, she would be more angry...LOL!
Cut her some slack and wait until she talks to you about it. If she knows about the changes that she will go through by talking to you or hearing it at school, she already knows...she is just angry and scared that it is happening. Nobody ever thinks ANYTHING will happen to them. Let her have her space and time, leave the pads. When she realizes that it will ruin her clothes to have blood on them. She will start wearing them.
I would just tell her that blood doesn't come out of clothes. You are not a fan of the monthly visitor either. And tell her when she wants to talk about it, you will be there to listen.
Many Blessings Mom!!!