First, have you checked with your school regarding the policy? (Does she want to go I suppose is the first question; been awhile since I was in high school) Some areas of the country are more friendly regarding home-schooled kids. Our district here is Arizona would allow it, again check with your district first.
I'd ask the school too. I know when I wen't to HS, most of the schools in our area allowed the student to bring a guest not from that school. The only thing that was required was the ID of that other person.
"Look Lois, the two symbols of the Republican Party: an elephant, and a big fat white guy who is threatened by change."