If you ever have to see a dr for any reason tell them in private that you will accept blood and to put that in your file. I am assuming your mom & dad will allow you in dr visit by yourself at your age? Tell them (dr or nurse) quickly about the cult and that you don't agree and to not mention it to your parents but you want that information in your files. If the worst case scenario happened the hospital would fight for you.
Do you have non jw relatives you could tell as well? a trusted adult at school?