I vote for hanging in there until you are 18. Nothing good can come of disassociating in your particular circumstances. Start making friends outside the organization if possible so when you do make a break you have something. Start planning for how you will pay for college, see a school counselor who can advise you what financial aid may be possible. Usually local community colleges for the first two years are the cheapest way to go, but even that will be a stretch if you are kicked out and have to live in your own. My grandson faced some of these issues, he wanted an expensive private college which no one was willing to pay for. He chose to join the Air Force, four years in he is getting out soon and has money for college, some credits, some useful job skills and much needed maturity and can now go to his choice of school.
Be grateful for your grand parents, even if they are in a cult, they at least care about you and put a roof over your head and gave you some stability when you needed it. If you handle it right they might eventually accept your decision to leave, who knows?
Being free mentally from this horrid religion is a great start, you can do anything you want in life. Good luck!