As a parent I wish you good luck and happiness in school. I hope your school is large enough that you make lots of accepting friends. My kids and their friends all seem to be pretty accepting and they are still in high school. There is one popular m to f girl in the school. I bet you’ll find you are not the only trans there. Personally I think you should give it a couple of days for your roommate to get to know you as a person before you drop the truth in her lap. I can’t understand why this would be a big deal. If it is go to housing and they will help you make a room change.