I had a few thoughts as I read this thread, and while not all of them might apply to you or your specific situation (as I learned reading on), I feel like I should share them.
Before nursing school, I had a serious desire to work in the ED. It was the goal the entire time I was doing my pre-reqs, nursing school, capstone, etc. I was completely driven to be an ED nurse. I knew going into capstone placement time that there were 2 spots and 20 people trying for the ED. I did not get placed in the ED, and I remember being pretty upset for a bit. I did get placed in an ICU setting where I learned quite a lot and ended up loving it, but I kept driving on toward my goal of working in the ED. I feel like my story may be somewhat similar to yours. Spoiler: I did end up getting into the ED new grad residency program at my hospital!
I know you ended up explaining that you weren't trying to put down your classmate, and I definitely understand the feeling of someone else getting what you want (and have wanted for a while). Do just watch how you word things, even if you're upset. There is so much of "politics" in nursing, and in the ED there are many strong personalities that have no problem repeating something you said while upset and turning it into gossip. There is also a nurse in my residency class that came in with a 'chip on their shoulder/something to prove' and trash talked people (including myself) to try and big themself up..... and almost no one wants to work with them. The ED definitely requires teamwork to function.
I also saw that you decided not to talk to your school about your placement, and I definitely agree with your choice. Just like what I said about the ED, school is another place where even a misspoken, upset conversation can hurt your prospects. I don't know about your situation, but my hospital is a teaching hospital linked to the school I attended. All the professors and school officials work at the hospital, and word gets around. If you did try to raise an issue about your placement, I would not be surprised if the CVICU got wind of it. That is not the way you want your career to start, even if it's not a paid position and you're not an RN yet. Even worse, I wouldn't be surprised if the ED heard about you not getting the placement you want and "kicking up a fuss," and their first impression of you would not be a good one. A school classmate of mine managed to destroy any hope of a career in the area by literally crying to the Directors about everything she deemed an injustice (a.k.a. not what she wanted), even if it was a normal part of nursing school.
You also mentioned that you don't know why standards for these placements are set but not followed. It's extremely hard to know exactly what they base their decisions on. Maybe they saw something special in your classmate during the interview. Straight A's do not necessarily correspond to great nurses, and C's do not necessarily correspond to poor nurses. I know it's hard to see right now because you're in the middle of school, but nursing school only gives you so much help in the real world and it takes a lot more to be a good nurse.
Really, most of my rambling post thus far comes down to "watch your mouth and be professional, you never know who may be listening." It's true, though. Best of luck!
P.S. In my ED interview, they asked about my capstone experience and what I'd picked up in the critical care setting! We do see a lot of sick patients in the ED, especially if the ICUs are full and we have to keep them with us for a while. Learn everything you can about ICU care and cardiac care, and bring it to the job interview with you (appropriately). Tell them about what makes you stand out learning-wise without being arrogant, that's how I got the job.