Someone might move in if you have Fill Empty Homes selected in Game Options. You can just move the other sims out with Manage Households if that happens.
Anything you left behind should still be there if you sold furnishings when you moved out. You probably can't go back and get things while you are at University, because they don't belong to you any longer. However, they'll still be there if you move back to the same lot. (I did that with one sim)
Everything from the house should be in your household inventory if you didn't sell the furnishings. However, you can't use your household inventory while you are living in a dorm, so you will have to wait until you've graduated and moved back to a house to get whatever you left behind.