In my experience, most investors only take a startup seriously once all principals (and they want more than one, and typically less than four) are 100% committed. A startup running on all cylinders means all people are extremely serious, have put serious skin in the game, and if they are kept very busy -- the startup's gotten good traction.
Partners not fully invested with their time shows a startup that isn't "hot" and/or bringing in those impressive numbers of eyeballs and early adopters/customers. If the startup was doing well, the guy in school would have to quit to give this burgeoning thing his full attention so it wouldn't fail.
More money is the goal of an MBA - of all degrees - and if he's still in school that means his startup isn't keeping him busy enough.