How would a girl feel about going to prom with her best friend?

Absolutely. I think a girl would feel more comfortable going with a good friend rather than going out with someone totally new. Unless this girl has an eye for some other guy as her prom date, then don't hesitate in asking her. I also think she will be less self-consciousness and therefore will have a really good time. And since your already friends with her it will be easier for you and her to hang out with a bunch of friends for after-prom party. Besides, what would be more beautiful than to see your best girlfriend all dressed up in her prom dress! Also, tell her exactly how you feel on prom night (of course don't be completely drunk otherwise she might not believe it). Have fun!

A girl would love to go with a best friend because is just a dance really that's all it is. Going to prom does not have any strings attached in and of itself unless someone attaches strings. Therefore prom is one night. So asking her to prom isn't the same thing as telling her you like her. In her eyes obviously you like her if you 2 are best friends. But to say you have feelings for her may complicate things so I would wait till after prom to tell her you like her