From understudy to Juliet? Help!?

I'm not in drama, but when the drama dep. announced that they're doing R&J, I had to audition. I did well, got called back when only the elite actors got called back, but, due to the deficit of substantial girl parts, landed with Lady Montague, who actually only has 2 lines. I'm also dance captain for the Capulet ball scene. But anyway, I was requested to be understudy for Juliet [this girl is a senior, has always done drama] I'm a junior, btw. But basically, for some reason, she's missed an abundance of rehearsals and STILL isn't off book. I had to sub for her time after time, and I AM off book. And - we premier in a WEEK. Plus. Her and the Romeo actor have a BAD history and hate each other... it's a disaster! I want her part more than anything in the world, and she seriously...I don't even know. Is there anything I can do?