Don't send a letter of disassociation. Nothing good can come from that.
Spend the next year of your life getting things in order. Definitely talk to your counselor/academic advisor and lay the ground rules with them that it needs to be confidential, then explain your situation. Work on getting financial assistance for college. Concentrate on your grades and DO NOT neglect making plans for college.
I can't stress this enough. Do not listen to any Watchtower idiocy about how getting your education is wrong. Just do it.
I know it's easier said than done. I was kicked out of home at 16 years old by my dub parents. Spend time now setting the groundwork for the rest of your life.
Do the absolute bare minimum you have to do to get by with your relatives who are still captives of the religion. You will hear complaints about not being super motivated in the religion. Stay the course, keeping your eyes focused on your real future, not the Watchtower's made-up fantasy.