My HS didn't have proms, as there were too few of us to make one practical (17 girls and 15 boys -- and the boys mostly wanted nothing to do with us girls). We did, however, get to choose the songs used for processional and recessional at graduation.
I went to a small, Quaker-run school that was very much into concensus. Our class had never agreed on anything in the four years I'd been there. We couldn't agree on class rings (one kid rigged the vote in his favor by making sure it was held five minutes before everyone arrived), or even our processional (half the room wanted Stravinsky's "Firebird Suite," and the other half had never heard it and wanted to before approving it), so the faculty or one of the bossy kids picked Vivaldi's "Concerto for Lute and Strings in D." I don't know who picked it; I just know I'd never heard of the decision till I saw the program.
The recessional, however, was picked unanimously by all of us. The grown-ups were so glad to see us agree on something that they let us use it.
All I will say it was both "appropriate to us" and "probably inappropriate for our parents and teachers."
So...can you guess what it was?