You may be the coolest 11 year-old alive! I don't see why you would have a problem with the 13" personally but whatever works.
May I make a suggestion though... Because of your age I would suggest the MacBook Air and here's why: weight! Believe me, that kind of weight will affect your back, and if you plan on carrying your MBP to school you will notice it. I have backpacked over 1000 miles so far and have royally screwed up my back from carrying too heavy a load for my size/age, that mixed in with 10-12 years of Ju Jitsu and Filipino Martial Arts can really cause problems. I applaud your drive in this matter, but at your age there is nothing that the MBA couldn't handle. The X3100 will do most graphics work in a snap and you probably aren't using any really processor intensive apps (i.e. Photoshop CS3, Maya, Final Cut, Aperture).
And not to call your parents liars, but I don't know if they will actually let you do it. My parents did the same thing to me with an Xbox, a guitar (I got it eventually) and multiple previous attempts to purchase an iBook and then a MBP, but never allowed me to buy one. I wish you luck, and if you don't mind me asking, what do you do to get the funds?
if you think the difference in weight between an macbook (or a macbook pro) and the macbook air is actaully going to screw up your back (if you are even calling the macbook or the mbp heavy) then you must either be a) very weak b) extremely misinformed of the weight of the macbook or mbp