No Case
I use my 13" at home and at school. I have a full body invisible shield on the top and the bottom, but otherwise just keep it in my backpack with my books, no separate anything. It makes it much quicker to access, and when I'm moving between classes I don't even put it away, I just shut it and carry it to the next class.
The case is made out of aluminum, a good rule of thumb is to not have it next to anything sharp or harder than aluminum! (i.e. no metal: gromets, keys, scissors, etc)
I love having such easy access to it, but I will say if you cary it in a backpack or another bag, be careful of putting it in magsafe side down. It is possible to have small bits of metal attracted to the magsafe port, which gets stuck there and when you plug the plug in, it can ruin the port!