Hey there, As his girlfriend, I think you have every right to bring it up with him...just be careful how you do. She is his friend, so chances are if you go in too strong and start attacking her he might get defensive and fiht against you.
My advice would be to bring it up gently, obviously in your own words and bring up your own concerns, but you could start by saying that him and *insert friend's name here* seem to be good friends, close friends. I'd make sure I told him that I understand that they are just friends and nothing more, but you think she's crossing the line by getting so physically close and that it's making you uncomfortable. He might worry that you're trying to choose his friends for him, so it'd be a good idea to reassure him that you're not.
After you do bring it up, you are going to have to deal with his reaction. He might say you're being silly and forget about it, he might agree and tell the friend to back off, or he might even get mad at you for bringing it up. You won't know how he's going to react until you do talk about it, but if it's bothering you so much you really need to bring it up before it starts causing tension between you both. The alternative is that you could pull her aside and gently bring it up with her. Being another female she might understand how you're feeling, and if she's a decent enough person she will stop the behaviour.
In the end though, you may also just need to get used to it. It's all about trust. There will always be girls touching your boyfriend, in some way or form, and it's not always sexual. It doesn't matter if the entire school were desperately in love with your boyfriend, the thing is he's with you. He's taken the commitment to be with you and you need to trust him through this. From what you've described it's only her making these gestures, so maybe he hasn't even noticed it. We all know guys can be a little, err....slow, when it comes to hints from girls, so just go easy on him.
I wish you the best of luck, and if you need to talk feel free to send me a message.