I can't see any reason why everybody must be good at something. There's nothing about genetics which says that random genes will combine in such a way as to give them at least one useful skill.
How would we go about testing your hypothesis?
The hypothesis I proposed is too broad to prove easily. Maybe, we can start with a narrower premise. We can disprove "if you suck in English class, you are no good for college". A friend of mine, a maths genius, got a D in History class in college (without deliberately trying to fail) He had straight A's in all technical courses (CS, maths etc).
Not only that, in math (and programming, etc), you just "get it" and thereafter practice it to get the straight A's. In History, even if you understand everything, you'll always have to memorize dates, names, people, places, etc.
This why smart students do well in school without having to study too hard, but maybe get worse (or even better) grades than the not-so-smart students who study their ass off and memorize everything.