Here's a little lesson - bullying doesn't stop just because we're adults. It still goes on, in the workplace, in social circles etc. It's sometimes more subversive than it was in childhood but it still exist. If you're still in high school and can't wait to get out so maybe the bullying will stop or you can be around adults, don't bet on it. It happens in all walks of life. Sorry to say, but this is the truth. Bullies don't grow up...they just get older.
they recognise that it isnt a common problem due to us all being adults but it is still happening.
You still might go to your boss or supervisor about it. Just explain to him what's going on and that it's creating unnecessary stress in the workplace for you and others as well. Explain that this girl is nice to your face but other people keep telling you about things she says behind your back and that there is obviously a problem, either with the girl or with the people who tell you what she says. Maybe then your manager will keep a closer eye on what's being said by whom.
But the problem is, i have no proof. She has never said anything to my face, she is dead nice to my face then as soon as i am gone she says nasty things about me to my best mate.
This is just a suggestion though. Obviously you know your workplace better than us and you know whether this would actually help or hurt.