How do you get them away?

Hey, want to go for a walk with me?"
This would let her and her friends know you want to talk to her alone with out saying the word private. When you say something like, lets go somewhere private so we can talk it sounds creepy or her friends will think you are shunning them. If you just ask her for a walk it is less threatening to her and her friends. Some b*tches will not let go. If they are ugly and bitter they will often times do anything to block your c*ck. Good luck with those chicks. If you aren't wanting to talk to them they will never let you talk to their friend.

It's a patience thing, if you're not just going to call her over, then you have to wait until you see her alone. Can you handle it?

if you were in a bar, I would say walk up to her, ask her for a drink and if she's out with a mother hen. then watch out for her. the mother hen never loses her chick.

but your under 18, so I'm assuming your at school, ask her if you can talk to her about a class you guys have, or and assignment you guys might have

Looshius said it all. Just be direct, ask her to walk & talk with you. She'll appreciate your show of confidence, but be sure that you steer clear of deserted alleys so you don't freak her out. ;) Pussyfooting around and trying to play games will only ensure that you lose her to someone else.

Nothing wrong with doing it directly. Tell her what you want to do and why "Let's go for a walk and chat a bit," "I want to get to know you better," "Let's get some privacy, I have something super serious to talk about with you." Make it playful, fun, light, you should have no problem, assuming the girl has an interest in talking to you. If it's a new group you don't know very well, you might need compliance from a couple of her friends, such as "You guys don't mind if I talk to your friend for a few minutes, do you?" Then just find somewhere to talk/makeout/whatever with her.