[lux]

Ask her out on a 'date'. But really informally. Walk around or a while talking, then try to get to a point where you're both sitting or laying down looking at something and just talking. Then say "You're a really beautiful girl, so I know you've probably been asked to prom already, or most likely will. But I really like you, and I hope to see you at prom." Analyse how she reacts, if she smiles and looks at you when you call her beautiful, good start. If she keeps looking at you when you bring up other people and prom, it's going well. Then if she's still smiling and looking at you, and appears to be thinking by the time you've finished talking, then ask her.