using the ipad with a bluetooth keyboard gives a student the ability to:
1. carry about the same amount of weight and bulk as an 11" MBA.
2. enjoy about twice as much battery life as the 11" MBA.
3. pay a little more than half the price of the 11" MBA (depending on what model you get)
4. use the ipad as an ereader, thereby completely emptying your backpack of books
if most of your time is spent reading, writing, or taking notes, then i think the ipad is ideal for a classroom environment. obviously, it is limited, and lacks a lot of the flexibility of a clamshell computer. if you have to choose one or the other, the MBA is the obvious choice. but, if you have the money for both a computer at home (to do some of the heavy lifting) and a portable device, then i would definitely recommend the ipad (as a supplement rather than a replacement for a computer).
because it lacks the ereader capability and has such a short battery life, i think the MBA is actually a lot less desirable for students than it would appear at first glance.