If your friends don’t hang out with you because you quit band then they weren’t your friends in the first place. Also, it probably won’t matter anyway because you’ll most likely won’t be seeing most of them ever again in a year (sorry). I don’t get how you can miss band if you aren’t enthusiastic about it.
How many AP/college classes did you take in junior year? Seriously, 8 AP classes while participating in EC’s and working on your college apps. Do you plan on sleeping? If you drop Music Theory, you’ll be taking 4 AP science courses - the AP science courses are notorious for being the most rigorous and dense out of all the AP courses. You shouldn’t be taking more than two.
You shouldn’t be doing something just for the application. If you do not enjoy band then quit it. Six years of playing an instrument, Vice President of Mu Alpha Theta, and multiple accomplishments in your school band? I think it’s the opposite of what you’e thinking; it shows that you are extremely committed to whatever you do. Quitting band won’t make or break your application, either.
My advice: Quit band if you feel like you don’t enjoy it. One extra year won’t make a big difference so that time is better invested somewhere else. Also, lessen your course load. You are high-achieving but 8 AP’s (half being sciences) is surreal. You won’t have time to make a quality application or essay if all you’re doing is AP homework. APES is one of the most useless AP’s unless you’re going into that specific field of science (course subject is basically common sense). I think it’s redundant to take two physics classes concurrently - one is enough.