I very conflicted on this.
So my ex girlfriend and I left on good terms, still hanging out as friends. 
She graduated early from highschool so never got the chance to go. 
The other night we were chatting and I mentioned that I was going and she immediately asked if she could. 
Not wanting to immediately say no I said 
"I can see that could be fun!" (Mistake on my part)
After a little a while I tried explaining to her that  I had already had plans with a group of friends and that it would be awkward  since she doesn't know anyone there and I would have yo be with her the whole time. 
"As long as I have you." Then she kissed me (OOF)

I dont want to leave my friends and have to be with her all night since, well shes my ex. However I feel awful for getting her hopes up in the first place and dont want her to miss out on this experience.

TL;DR 
Ex wants to get back together and go to prom at my school. I already have plans but got her hopes up. What do I do?