Daughter flashing knickers deliberately

  1. For the last month or so my daughter has developed a pass time that I need to get her out of 

She's keeps flashing her knickers every opportunity she has every time we go somewhere she deliberately leaves her legs open!

I went shopping with her yesterday to get some Easter bits she hiked up her skirt on the bus and left her legs completely open their was some

Older teenagers opposite us and her knickers were clearly visible I could tell they were the way their eyes was facing I feel completely disgusted 

I quietly asked her if she could close her legs and she told me no I don't want to the whole bus ride made me uncomfortable knowing her knickers were on full display 

I wouldn't mind if she had shorts underneath but she's not just wearing normal knickers she's wearing thongs

Is their any thing I can do or say to stop her from doing this?