Yes, thats true, some people might suck at everything. But that can be managed as long as thats a small enough minority. (we have government welfare for a reason, right? :))

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).