Here are a couple things to keep in mind:
40% of your OML score is academics. If you are really worried about what branch you will go into when you graduate, don't go to MIT and major in physics with three minors, unless you are really smart. find a school and major you will be academically successful with.
Bruno must be an academy grad. ROTC will give you the same leadership experience, but the difference between ROTC and academies and SMC's is your day to day life is not structured, and you'll have to figure out how to solve the day to day challenges of time managment, and socialization, and how to solve out of the ordinary challenges on your own. There won't be a TAC or squad leader to tell you what to do every day. (can you tell I'm an ROTC product).
You will not be an Army Ranger right out of college, no matter where you graduate. If you want to be an Army Ranger you need to attend airborne and ranger school, and do well where ever you serve. Your best chance of making it to a Ranger Battalion is to be an infantry officer, and you won't even start thinking about a Ranger regiment until you are a 1st Lieutenant. There are only 3 Ranger regiments in the Army.
whether you have a desk job or not only has a little to do with what branch you are in. You will do many different jobs as an Army Officer, and the job we do is usually an outdoor sport. The personnel officer in an infantry battalion is usually a junior captain or 1st lieutenant who is wearing infantry branch. He may spend a little bit of his time behind a desk. The transportation officer, or logistics officer, who you would think would spend much of their time behind a desk is probably leading convoys and spending a much more exciting time than expected.
You are going to be a senior in high school. i would suggest you keep an open mind. Talk to as many soldiers and vets as you can, ask questions, and take everything they say with a grain of salt. A military career can be just about anything you want it to be, whether you are jumping out of planes with the 82nd Airborne, riding around in tanks with the 1st Cav, or engineering signal networks with the signal corps.
Best of luck.