It's true that currently, some intelligent people can fail to develop a work ethic because they've found things easy.
However, this is not an inherent problem of intelligence; this is an inherent problem with one-size-fits-all schooling.
They found things easy because the difficulty level of their challenges were well below them, but that's hypothetically easy to fix; you make their challenges a little above their current level. Practically, though, it's more of problem.
Seems like a good problem for a more software oriented learning approach.
I remember daydreaming in class wishing I could just go through the book myself and be finished with a grade in a few weeks. I did get this experience in math for a while when I was living in Texas. You didn't have to take any topic upon which you could pass a pretest, and then you got to take a test on a topic as soon as you were ready for it. I was able to zip through multiple grades until I reached some material where I needed instruction. Of course, once I moved up to the next school, I was back in the standard classes, daydreaming.
However, this is not an inherent problem of intelligence; this is an inherent problem with one-size-fits-all schooling.
They found things easy because the difficulty level of their challenges were well below them, but that's hypothetically easy to fix; you make their challenges a little above their current level. Practically, though, it's more of problem.