Girl's only please. Where to find a dress? Any tips for going to a formal?

In December I'm going to go to a formal with a bunch of my girlfriends. I don't expect to dance with any guys bc most guys at my home school co-op wont ask a girl to dance with them. But I still want to look nice. Where can I find a somewhat modest prom dress that will fit a 12 year old girl? It would be nice if I can get it under $50. Links would be helpful. Any idea's to what other stuff to wear, how to do my hair, nails, makeup, etc..? I am 5ft 3in and 95lb. I have long, very curly brown hair. I'm also going to get ready with a bunch of other girls so they can probably do my hair. I want a hairdo that won't come undone if I'm dancing. i want comfortable shoes, preferably flats. My favorite color is light blues. I want the dress to go down to at least right above my knees. thanks!