They're correlated, but they're definitely not equivalent. It's very easy to game your classes and be an A-/B+ student while learning basically nothing of importance. That said, smart people will tend to have good grades and be successful at work. In my experience, if you consistently show up and aren't retarded it's pretty easy to be liked and fairly successful in a workplace, but even that's asking too much of a good chunk of the workforce.
ime, the really smart kids (i.e. gifted) struggled in the real world because they went K-12 without having to work at anything, so they had no idea how to applies themselves in the real world. The ones who were a step below that - smart, not gifted, but were usually studyholics - continued to do well because they had built those work habits through years of practice.
Your last sentence is super accurate. The basic rules for keeping a standard jobs are:
1. Show up
2. Show up on time
3. Do something
An awful lot of jobs just need warm bodies. My job is primarily IT hardware administration, just about anyone with a knack for computers could do it, so it's definitely not special or anything & doesn't pay nearly as well as other specialties do, but I happen to really enjoy hardware & love that I can play with stuff & get paid for it. All depends on what your personal goals are & what's available. And if availability doesn't exist, being willing to move.