How to isolate a girl from her friend so you can ask her out during school?

there's this chick I really like at my school... I am a junior in high school and so is she. I want to go out with her and ask her to the prom.. I wanted to do it during a class where we both sat next to each other, but we just had our seats changed so that way of asking her out went out the window. I have a few other classes with her (don't sit near her in any of them) and I often find her staring @ me when I am not looking.. I want to approach her and just ask her out already, but she is always with her friends.. and I want to ask her alone.. so how do I isolate her from her friends? and should I ask her to the prom or to go out with me first?

thanks in advance.
How to isolate a girl from her friend so you can ask her out during school?
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