MBA vs rMBP vs Lenovo X1 Carbon - for business school ?

Being this is for school, I think you would appreciate the battery life of the MBA, especially for Excel primary purposes. I would do the MBA which would be cheaper than the MBP depending on features.

Depends

If your programs are mainly Windows programs and Excel is a big one, than i would stick with a Lenovo. You may run into more than one program that is Windows exclusive. Office runs better on Windows anyway. The MBA is an awesome product but you'll more than likely be putting Windows on it in a VM if you have any programs that make you use Windows. If you know what you're getting into i would say go with a 13" MBA with 8GB of RAM Minimum use it until you need Windows then do a VM. Of course this will add alot of cost onto the machine. So if you can't afford it (and as a college student don't buy stuff you can't afford just to have it) buy the Lenovo. I feel like you may have less headaches with the Lenovo just because the business world lives in Windows. If Excel is your main concern i think you might be ok. But what about Access or Visio or any other apps? Maybe ask your school's office or a professor or look at what's on a computer lab machine.

I'm a Desktop Technician for my school as well as the only Mac Tech and i just cringe when having to fix "Mac issues" because more or less the people should just not have a Mac (mapped drives, network printing, various scripts that run to keep them happy on a windows network, Mountain Lion being a pain).