I feel that it's your assesment that lacks common sense.
Introversion has nothing to do with achievement in academics. If anything, E types might get more distracted with their surroundings than I types.
N... no comment. I've yet to meet an N who would make it to university, really.
F/T difference doesn't matter at all, of course. Do you imagine that F types spend all day reading Harlequins or what?
P is the only parameter that can really get in the way, but not necessarily so. Js can procrastinate as well. In fact, my INTJ friend was the one who taught me how to procrastinate effectively.
I'm not saying that some types aren't more prone to be underachievers than others, but you haven't chosen the right way to determine them. Personally, I doubt that type would be NFP, because NFPs have Te, and despite the common misconception, they can often use it well. If I've met any underachievers in my life, these were STPs and NTPs, but 1) maybe they were "underachievers" only in my eyes, 2) definitely not all the STPs and NTPs I've met were underachievers. It depends on the individual's self-discipline, ambition and ability to use their intelligence in their advantage, and I don't think this is necessarily correlated to type.