Have you thought about telling your school councilor? Explain your feelings about not wanting to be a JW and your fears of what you parents will do if you tell them. See what he/she might be able to do. Perhaps have him/her write a letter to the WTS inquiring as to how JWs respond when their children decide not to be JWs. Odds are the WTS will respond with a very politically correct letter from the PR department... they want the schools to view them as "normal". Once you have that letter from the WTS that plays things down you might be able to show it to your parents to get them to cool their jets.
Excellent suggestions but I would keep them for a second stage (if it turns bad). Doing this as a first step could cause your parents to feel betrayed. Give them a chance for a smart response (even though it may not be the immediate response).
But sometimes it does turn bad and a third party is very important. When my younger half-sister was sixteen she told our father she didn't want to attend meetings anymore. My father beat her, dragged her down the stairs by her hair and locked her in the bathroom for several days, with her own mother encouraging him. When my sister finally could get out of the house, the first thing she did was getting to the minors Justice. There was an inquiry and an educative control decision. As a result our father had to step down as an elder. Any religious pressure was impossible from that point on.