Moe, I was twelve too----wondering the same thing. It's a shame-you want to belong so badly in middle school, it's such a bad time, especially if you're a witness. No other non-JW wants to even be seen with you, so what alternative do you have?
But, their argument would be is that you can leave anytime, they are not forbidding you from leaving-it's just the emotional and social consequences that will follow.