M.C.
Most likely she will definately say it's not hers. Fact is, most likely it is. I have a 13 year old, and he has told me that many kids at school smoke pot and that they think it's cool. I've started very young with all my kids about how the dangers of drugs and alcohol and how unacceptable it is, and I talk about it all the time, and show them many examples.
You should get the book "Beautiful Boy", it's a best seller out now, it's a memoir about this dad writing about his sons' addiction. His started with pot. Pot smoking ALWAYS leads to heavy drugs! Nip this in the bud now, it is very dangerous for her to start. Have her watch a couple of "Intervention" shows with you, those may scare her too, they are on the A&E channel.
Trust me, don't ignore this, it will get out of hand, and as much as you'd like to believe her, don't! Remember being a teen, it's a tough world out there!
Good luck!