Welcome to your new favorite forum! Telling a school counsellor is a good idea IF the counsellor understands NOT to relate this information to your parents. Actually they (the councellor) will not take any action unless your parents begin to restrict your school activities or lock you in a closet. Maybe explaining the harsh side of shunning will help the counsellor understand better. Even better tell your school counsellor about this website.
Get a plan. How are your grades? Are you involved in extra activities at school? I know right, how could you be with JW parents? OK well I'm just thinking of your college transcripts, extra curicular activities are viewed favorably. Do you live in a state which has funding for the top 10% of students? Focus on school. Tell your parents that you are thinking of accounting (that way you can help at assemblies etc) this will at least buy you a few years at home.
Good luck and please let us know how you are. I have received support here and I know you will too.