Me and sarah have been dating for 7 months now, and I glanced at an old e-mail she had sent to a friend. It had said
"...I can't believe we are going to be sophomores next year! We need to find someone to go to prom with just for the hell of it =p or maybe not... IDK if anyone would even think about taking someone as ugly as me though =P"
It was sent to her friend about 3 weeks after we had started talking, and about 3 months before I asked her out.

I'm a senior this year, and I'm pretty sure that we're dating for the right reasons, I just want to make sure that this looks very far fetched and too much of a stretch, I mean... No one would set up a relationship 8 months before hand just to go to prom, right?

Yes, you are right. This must be just some funny talk she must be having with the friend. You know, the lighthearted kind.

Talk to her about it