Here are a bunch of things for you to consider:
- Screen resolution. I don't do graphics design but I bet this is going to be a serious issue. When you start talking about print graphics the screen on a 13" Macbook would let you view roughly 1.5 square inches of whatever you're working on. There's a patently retarded trend in the computer industry right now that limits availability of higher resolution models in smaller form factors, so there's probably going to be a tight relationship between screen diameter and resolution regardless of what model you choose.
- Photoshop performance. This means having lots of RAM, and a decent CPU. It's fairly easy to come by.
- Gaming performance. This requires a high end graphics card, which can be difficult to find outside of desktop replacements and dedicated gaming laptops.
- Battery life. Adversely affected by having lots of RAM, a decent CPU and a high end graphics card.
- Physical size. A lot of laptop backpacks are designed for 15.4" at max. How big do you want it?
- Weight. Adversely affected by everything above.
- Durability. This is where about 99.9% of the price difference between a HP and a Thinkpad comes in. Do you want a laptop that'll last for 4 years, or do you want one that easily separates into two or more components?
My compromise was the Thinkpad W500: durable as hell, okay weight, okay physical size, good battery life, poor gaming performance and a very high resolution screen. It wasn't cheap. And I have an 8-core Mac Pro to do the heavy lifting, which also wasn't cheap.
In the end you're probably going to have to pick two:
- Cheap
- Good for school
- Good for gaming
