After having a couple of days for the immediate panic to subside I've had some time to think.

I know I really like this girl. We get along so well together, like each others company, and we just click on a fundamental level.
So, when we do meet up again, I'm going to do a few things.
Namely, I'm going to apologize. When she first breached the subject, the idea terrified me. As I said, I've never been in a serious relationship before and I was (and still am) scared of that kind of thing. I panicked. I told her that I didn't feel the same way and that I didn't think of her in that way. Which simply wasn't true.
I plan to simply tell her the truth. That I really like her, I'm willing to give a serious relationship with her a shot, and be honest about all the reasons I'm hesitant.

The two big ones for me are school and polyamory.

-If we do decide we want to go for it, she'll be home with me for maybe 3 or 4 weeks grand total. That's in between her going on vacations and attending summer school. After that, she'll be off to college again and I'll be staying and attending classes here. Now, it's not as if she'll be across the country or anything. In fact, she'd only be a little over 3 hours worth of driving away. Still, it's a matter of some concern to me.

-She has made no secret of that fact that she is polyamorous. Which, as I've said, I do not hold against her in any way. But I wouldn't be cool with it if we were in a relationship together, it would hurt me too much. I don't know how she'd take this. Sex is important to her and I admire that about her. But if that means that she'd want to have sex with others to fulfill needs that I can't or won't meet, then what happens?

Most importantly I don't want to loose her as a friend. We're both of us short on people who get us. We're both great for one another in that we are such good friends and we have such a positive healthy friendship. I'd hate for us to try and be more than friends and, if it doesn't work out, lose our friendship in the process.