Yes, it is possible to branch aviation out of USMA and go to flight school. It varies with each class, but 80-100 cadets get it each year. It's important to note, however, that the aviation warrant officer and the aviation RLO will have two very different careers. Warrant officers are the technical experts who fly everyday and know their airframe inside and out. Conversely, as a LT or CPT you'll be out front leading and flying when you're in a leadership position (Platoon Leader and Company/Troop Commander), but you'll also have to rotate onto staff, at which point you may just be meeting your minimum hour requirements.
Either way you go, you'll have a very good career in front of you. Both have their advantages and disadvantages though. I suggest researching both career options if that's what you're interested in.