(One of) the most important skills/attributes for starting up is the ability to hustle and be astonishingly flexible yet focussed at the same time.
Most (or at least, many) people in the entrepreneurial world don't have MBAs - because for many people entrepreneurship is about thinking differently and doing. Studying at MBA level fits into neither of these character traits.
This is not to say that if you already had an MBA you didn't learn something that would be useful.
If the question is really: "Where can I best learn to be an entrepreneur, at school, or not at school?" Then I would answer "not at school".