Madness? THIS IS WIDESCREEEEN!
Uhhh sorry I had to.
Anyway yeah, if you're planning on using your laptop during lectures or in a classroom, 12" is king. It's sturdier, lighter and easier to pack in with all your books and papers than a 15". It also leaves you room to have your notes out while having a screen big enough to take notes with and write essays on.
If you don't need to take notes with it during lectures or have to use it for coursework in class, I do feel a lot more comfortable with 15". My friend has a 12" HP and it's a nightmare to do research with, there just aren't enough pixels on the screen to display two documents side by side. It's just as easy to lug around in a backpack, although it is a bit heavy to leave hanging on the backs of chairs. My bags are no bigger now than they were before I got the laptop.
Individual tastes vary, so I'd at least get a hands-on experience on a friend's laptop, maybe see how they carry and use theirs etc.