Yourroommate needs to be told ahead of time. Why should you get to be the only one in a position to make choices on your living together? You deserve respect but so does she.
If this was a matter of race or religion or the like it would not be such a huge issue. But being assigned a room with someone who identifies as the opposite gender is. Roommates often dress/and undress in each other’s presence; rooming with a guy could make her very uncomfortable. This isn’t prejudice against you for being transgender but just not wanting to live with ANY male. Being assigned to you is no different than being assigned to anyone born genetically and psychologically male.
As to your other question how would they not notice. You will be living with a woman in presumably a women’s dorm or dorm area. You will be presenting as male. Yes that will raise questions. NOTICING is not the same as JUDGING however. Many won’t care. Some mightbe curious. And some will be prejudiced. But you are living in a pretty accepting generation so you will most likely be accepted for who you are as a person as a whole and not just gender identity.