I think that the "B students" know who they are: they are the students who are not so fond of math / physics / litteratures, etc.. but have other interests in life.
The idea is not to force these guys to learn entrepreneurship, but rather not to force them into studies that don't fit their qualities and potential. Most of them would be glad not to attend the hardcore math class and instead do something useful that interests them..
I think the term "B-student" is just an effective way to identify to the reader students who wouldn't do well in math or science for any reason, whether environmental, motivational, or natural. That is, it brings about a connotation of someone who is smart enough to succeed at most things, but who can't compete head to head with students who specialize in a specific academic field.
It's hard to define, but the term's usefulness comes from its general connotation and the associated imagery, not its definition.
I think the takeaway here is that these individuals, whatever you call them, should focus more on learning skills that will allow them to compete laterally by creating opportunities for themselves and others rather than learning to be superficially cultured or trying compete for existing opportunities head to head with others who are more specialized and talented in their respective field.
"B school" == "Business school"