I'd highly recommend going for the iMac + MBA, that's what I'm going to do this year. Of course, that's just possible if your budget allows you to spend that much money. But you can't beat the power of an iMac and the portability of a MBA, with this combination you'll be able to do everything. In addition, the MBA isn't that weak, video editing "on-the-go" is possible too.
Keep in mind, that the MBA is going to be refreshed very soon, likely it will get Sandy-Bridge processors -> extra boost in speed.
With the new iCloud-system becoming avaiable this fall, it will be no problem at all to keep these 2 devices synced. Even without iCloud it isn't that much work to use the new "airdrop"-function in OS X Lion -> you don't even have to use a USB-Stick.
I can't recommend buying an iPad instead of a MBA (in addition to your iMac). You can't use the benefits of a whole OS on the iPad. Let's say you're going to do photo or video-editing on your device with a program that you've got from college -> you'll likely won't be able to install it on your iPad.
Plus, The iWork and iMovie -Apps on the iPad are light-versions, with the MBA you get the whole version. Typing on an iPad for hours is not very handy too. On an MBA you are able to use USB-Sticks, if you have to copy stuff from your MBA to a friend in college. No chance with an iPad on that.
If you don't have enough money for 2 devices I'd recommend buying a MBP. Keep in mind that a MBP has a DVD-Drive, a MBA doesn't (well, you can buy an external one, but why you don't buy a MBP in this case?). The MBP is heavy but if you're looking at processing speed, the MBP is almost up head to head with a base-model iMac.
But if you have an iMac at home, don't go for the MBP, the MBA is just super-thin and works fine for taking notes, surfing, photos, videos, etc. It fit's in every school-bag, and it's even lighter than most books. It has a 16:9 display, a FULL-SIZED keyboard and a multitouch trackpad. With the upcoming ugrade do SB-processors it will hopefully get a huge boost in processing-speed and maybe the battery and storage are getting even better/higher.
To sum up: Go for:
iMac + MBA
or
MBP
An iPhone or iPod could be helpful as well. I found a lot of nice Apps for highschool (like dictonaries for biology-tests or translators for French-tests). I think it's important, that you can takes photos in class and add them to your notes later on. I bought (with my iMac) a HP printer/scanner too - you'll need a printer and a scanner for 100%. Especially when you're going to work with a lot of books or papers you will need a scanner. With a scanner you are able to work with photoshop etc. too. A printer+copy+scan will cost you around 200 bucks if you order them with your Mac. I can highly recommend to buy one.
Nevertheless, no Mac or iPhone is able to replace your brain
Good luck in college !