I know exactly how you feel. I told my parents I was bi and got the responce "Are you sure? All you have had was boyfriends." Of course we are sure, and we've known this ever since we could have a stable thought.

So what I did was say, "Why would I joke about something like this? Oh yeah, I definately want to be made fun of at school. I have known for a while mom/dad and was just to nervous to tell you guys because I was expecting THIS response."

They then apologized and things got better.
I hope it does for you.