As others have suggested, I would HIGHLY encourage you to stick with free software/low cost software for now and save your money for a car, or heck even investing it for possible future software needs when you get out of school... Things change for many people during the course of their high school life and you may end up not wanting (or not being able to) afford a living as a graphics designer.. With that said.... since that's the direction you want to go for now, buying a tablet as soon as you can afford one will help FAR more than any software will IMHO. Especially since almost all graphics/illestrative design will be able to support tablets...

Now... once big thing to think about is if you end up going to college or toward the end of high school, Adobe offers HUGE discounts for students and that would be the time to spend the money if you decide to keep going that direction.