Can I give a bit of advice? No? Well too bad, I'm giving it anyway.
In my general observation I would actually advise NOT taking any girlfriend or boyfriend to your Yr 12 Formal. I know this is strange but hear me out.
For my Yr 12 Formal I was one of the DATELESS WONDERS (not being able to maintain a lengthy relationship and too afraid of commitment to ask my current crush, and that is all it was, for the date even though she went to the same school and was in my year). However, I am VERY happy about this, as it allowed me to enjoy the Formal as MY night. Furthermore, I won't have any bad photos of me and an ex-girlfriend should we have broken up after the formal (my mate had that scenario, and all photos were him and an ex-girlfriend).
Why don't you tell your girlfriend to go to her formal alone, and you go to yours alone, and meet at the afterparty. That would probably me the easiest thing to do. And if she is unhappy about it, can you see a relationship like that working out?