I would fully expect that your roommate would consider it rude...and saying that it's because you want to be alone won't help at all. Room dividers only work if it's a mutual decision.
Your best bet is to follow Ai_Ling's suggestion. If you want to be alone, don't get a double room. I remember that I had the same concern when I was starting university - hence, part of the reason I chose the school I did is because all the rooms are single bedrooms with a shared bathroom for every pair of rooms. A much better situation - still technically had a roommate, but had my own space.
Honestly, from the sounds of your situation, and especially given that you're going to a known party school, I say avoid a double room at all costs. Even if you have to get a room off campus to find one, and even if this is a little more expensive or less convenient, do it. If you want to be alone and value privacy, a double room is not a good idea.