I am assuming you are in some kind of boarding school? Is this in the US or elsewhere?

In any case, it seems like you are rushing into things; two weeks is way too fast to decide anything like rooming together or deciding on who your closest friends are at a new school. Where is the girl you met during orientation? What about her friends?

Why not just step back and not initiate conversations with A? Be friendly, just see what takes place.

Well, I don’t think we decide who we like. However, you can choose how much you invest in relationships with people, and how much it is worth it to do that. In my experience, if you take a step back from the situation and don’t make an effort to invest in a crush or to “win” them over, so to speak, odds are the attraction will also fade.
Sometimes that is a necessary step when you have an attraction to a person who is also a friend and you are unable to date for whatever reason.

I agree with @DigitalBlue . I would not cut myself off completely from that crowd but I would look also to make and spend time with others outside that crowd and reduce your contact with A. I was recently in a similar situation and have reduced my attraction to my “A” while still maintaining something of a friendship by investing more in other people and activities.

Why would you waste time on someone with whom you have been shunted to the bottom shelf? Move on and find someone who puts you at the front of the line.
(Or focus on your studies and rock the degree, get a fabulously paying job, and laugh while they still try to impress each other with beer bongs ten years from now!)

Avoid A at all costs. Definietly do not room with her. You need to get over her and seeing her all the time will not help you with that.

She has moved on (she did lead you on) and so should you.

If it’s clear this senior guy is using her, is it not also clear that she is using you? Do you want to be used? Once you know you are being used, it becomes your choice if it continues.
This yo-yo texting is a bunch of crap, imo. No one deserves to be treated like this. The way I see it, she is way unstable and not someone I’d want in my life. You can distant yourself without moving and without losing your other friends, you just have to choose to do so, make the decision and stick with it.
Never make someone a priority in your life when you are only an option in theirs.