I usually don't post long messages here, and you probably don't know me from Adam, but consider these four items, QuothTheRaven:
1) Do the simple math: Do you want to go to the prom, and if so, with a date? If so, you
had a slam dunk. You can't assume that asking her again will be a "yes." Still, if your personal requirements are to go with a date, then this is likely your best shot.
2) In theory, as soon as you graduate from school, you will immediately recognize how utterly ridiculous the peer pressures are. You said something along the lines of "any guy in my class who knew her..." Be your own individual, and make your decisions based on what
you feel is right, not what you think all the other guys would do. That's peer pressure, even if you can't see it.
3) This moment could haunt you. At my 10 year high school reunion, I was approached by a school friend I hadn't seen, and she asked if she could apologize. I asked, "for what?", and she told me that she still felt bad for not paying me back $5.00 she borrowed. I never remembered such a trivial matter, but it bothered her for months leading up to the reunion. Go figure! Be sure to take steps to "make this right," whatever that means for you in this instance, so it does not become baggage. Will you feel bad seeing her at your reunion, knowing you turned her down for the prom because you were unsure?
4) From the fact that you've struck out twice, I would invite you to consider the possibility that you are not the catch you think you might be... while that might raise your hackles, please accept it in the simple way it is intended. If girls aren't tripping over themselves to ask you out, then it's pretty clear you're not one of the top prizes in your school. You might not view it that way, but the girls seem to view it that way.
As a possible alternative, have you considered actually getting to know this third girl a little better? How about inviting her to some activity one afternoon, so you can get to know each other just a little more? Just make sure it's not her with you and ten of your friends, or the other way round.
Spend some time to get to know her, even if you don't go to the prom. In that way, you can know for sure.