I know this post is old, but i was creeping on here and found it and my situation is quite relevant.
I go to a school on a quarter system, so we have finals in about two weeks. Well my Top school invited me to an interview during finals week. unfortunatley I had to contact them and tell them that I could not go due to the fact that I was taking finals, and asked if I could reschedule. It happened to be an interview weekend, so they couldn't reschedule, but told me they would try to set up a phone interview or Skype interview. I've been worried since then that I ruined my chances, but if I had gone I would have failed my classes, and wouldn't have been able to go to grad school anyway; so it was sort of a lose-lose.
just thought I would vent. I hope everything worked out for the OP