Thank you everybody for your comments. I've decided to use both across four different shirts. I have them set up at my store, and you are welcome to check them out and see what they should look like.
Now the thing that came to mind as I was finishing up was, "Do I actually have a legal right to make these shirts, or might I get sued by the School District?" I honestly have no idea. I would think that for a limited run that isn't being sold for profit I might be okay.
Therefore I've considered two options to avoid getting sued. The first is the shirt will only be available for a limited time (up until I get mine ordered and in my hands) and the price is set to the minimum level required by CafePress to cover their costs so I won't be making a dime from them.
The second thing I'm going to do is, after my son's birthday, I'm going to offer to give the school the artwork – with slight modifications to the color so my son's remains unique – on disc and the rights to use the artwork for any fundraising purpose they want free of charge for the next three years. After that point they will have the option to buy the artwork outright or stop using it.
Anyone have any other suggestions or advice?
Since the eyes were remarked upon in the comments, here's a close-up of the eye by itself:
The convenient thing is, because of the way I built it, it's very easy to change around to suit my needs:
It's still not quite as good as I would like – it needs to be a bit more random, and less patterned in the lines – but it'll do for now.