i've had the 13 inch , it was the first 2008 aluminium mbp, with the use of spaces you can really maximize your efficiency to the point where the less 2 inches won't make too much difference, if you are wondering what that is it's basicly having separate screens in your computer, each one containing the windows you wish, for example, firefox on space 1, and word on space 2. switching between them is has fast has hitting the binded key for it, in my case it's cmd 1 , 2 , 3 , 4 or the arrow keys.
but now that i've got the 15" 2011 i'd never go back, the screen is bigger and yes it's heavier and more bunky to carry it arround but the extra space is really worth it.
i think it's more of a personal decision you must make, i'd sit down with the 13 and play with it for afew mins, browse the web, then switch for the 15 and do the same.
if you are wondering if the 13 inch will get the job done, it will thrust me, and with time you'll get used to the smaller screen and not even notice it. when i had it i'd use to look at other people's 15" and not see much difference.
don't forget that the 15" comes with quad cores instead of core duos, but if you're using it for school i doubt you'll need the extra firepower it brings.
hope i helped and didn't confuse your further, keep in mind that having the "higher res" screen, like i do, makes everything smaller giving you the impression that you've got more screen so my opinion might be tainted a bit.