As a current law student, I cannot emphasize this enough:
GET A SMALL, LIGHTWEIGHT LAPTOP!!!!!!!
You want a full-size keyboard. Decent battery life also might be helpful, although most schools have outlets everywhere. Beyond that, I'd sacrifice just about everything else for size/weight. You're going to be lugging this everywhere, and will be using it mostly for low-intensity uses (typing, emailing, IMing). I went for performance/screen size/etc over weight/size and terribly regret it.
Really? I'm sorry, I have such a hard time giving up a cd/dvd burner. It's really not necessary? Do these really small/lightweight ones have USB ports at least?
I definitely think you want a CD/DVD burner. There are plenty of computers on the lighter side that have them. Also, you can either get a "docking base" that makes the computer bigger when you need a CD/DVD burner (a la the Thinkpad x61s that Pikey mentioned) or have an external CD/DVD burner (a la the MacBookAir). Lenovo is also coming out with a new lightweight portable--the ThinkPad X300.
I'm not saying you have to get an ultraportable. I'm just saying you probably want a 12-14 inch screen rather than a 15-17 inch screen. You don't need tons of extra options, since every little thing adds weight. Just look at the weight info. Definitely keep it below 5 lbs, but the closer you can get to 2 lbs the better.