I am a firm believer that dreams cannot predict the future. Quite simply, the future is the future, and we cannot predict what cannot be predicted.
My inturpretation of this dream is this: your fears of being rejected when asking her to the prom are being manifested in your dreams. Her not recognizing you is not all that dissimilar from her not accepting your invitation. With senior prom coming up, and the dreaded (yet anticipated) departure to college, it will be your last chance to hook up with her before going your separate ways. Your subconscious knows this; you know this.
If you have feelings for her, then go for it. If you want to take her to the prom, then ask her to the prom. Dont let your fears and clouded mind hold you back, you'll only regret it in the years to come. Take my word on this - as your position sounds far too familiar to me.
Of course, this is only my inturpretation of your dream. Which is based on the small amount of information you provided. Obviously there is a great deal I dont know about the situation, so only you can decide what is right.
Good luck, and my parting words are this. When you're done college, and you bump into her, do you want to think back and say "damn, I wish I worked up the courage to ask her out" or would you rather say "I remember when I asked her out, and got shot down... damn that sucked." I'm not saying you'll get shot down, I'm just asking would you rather live with that doubt, or just find out now?