I had a hard time comin' up with comeback statements at that time in my life as well. I would just "freeze up" mentally and not be able to react by throwing a comeback statement at the individual. I once grew a pair when I was eleven and made a comeback statement to this female bully on my basketball team. This is how it went:
Bully: "Your hairdo looks retarded!"
Me: "Not as retarded as you!"
I HATE the word "retard" <_<, so that just made me more upset. BTW, she was not used to anyone, especially one of the most quiet kids in the school, talkin' back to her, so she shut up

. I did pay the price a few minutes later during our scrimmage, when she ran full-speed (I was on the offense) and slammed right into me, sending me ten feet before I slid on the floor. Considering our different proportions:
Bully: 5'4", 150+ lbs, plus size.
Me: 5'2", 90-95 lbs, size 3.
I was seeing stars

. Oh, well. I'll never understand why some folks, although they know better, feel they need to bully others to get their point across

:.