POSSIBLY MY LAST GRADUATION BALL!!!, and I do not know what to wear!!!?

Go with something cly, sophisticated, and if you're going to be wearing a mask: dramatic. Hour gl figures go well with almost anything. You can do all sorts of funky prints or something with real simplicity. To look tall and lean: black. It's slimming, always flattering, etc.... That's all my advice, hope it helped and good luck! ;)