I use the 11" Air in school for note taking, browsing, itunes, etc. Never has the battery got lower than 45%. I think it will last 5+ hours straight working a medium load, no problem. If you're taking classes one after another and need a laptop in each and need to work on it for 5-6 hours straight between charges, you pretty much need the 13". I use mine quite a bit like I said. In the end though you just have to add up the pros can cons, check what your gut is telling you can commit. If you make a bad choice, you have 14 days to rectify it.
Also, the 13" is small, portable and easy to carry but the 11" just disappears inside a bag or purse. I write and I like to have something handy at all times to write on and the 11" can go anywhere and everywhere, and places the 13" might not go as easily. You get used to the screen. If you can use an Ipad, you can use a 11" Air.