> Maybe try to figure out how to make the work fun?
I don't think this is the right approach, because you need the ability to focus on work even when it is not fun.
You do need something that makes the work rewarding in some way, but I wouldn't push to hard to try to make it fun.
> Or, try and instill a goal less of "getting the grade" and more "attain mastery of the subject"?
This was my approach in high school as well. It led to me being the one everyone wanted to study with because I picked up the material the best, but then I flunked classes because I never did the homework.
I would flunk classes that depended mostly on homework, and ace classes that depended mostly on tests. Since I had convinced myself that mastery was more important than grades, this reinforced my behavior.
Somehow, the acing the tests part allowed me to get into an Ivy League college. Which I then flunked out of because at that point, I couldn't get by without actually doing the homework and studying outside of class.
This pattern of behavior continues to be something I struggle with as an adult. I can be quite productive for short bursts of time, but have a hard time keeping on track with longer term projects that don't pay off immediately.
I think what you need is to learn the value of steady incremental work that only shows significant results after a long period of time. Basic reinforcement training techniques can be used for this; provide rewards for getting incremental work done, but better to provide them intermittently. Eventually work up from smaller rewards for smaller goals to larger rewards for larger goals.
I think that reinforcing consistency, habit, and small incremental progress rather than brilliance is a big part of what you need. The problem with brilliance is that you can easily get a lot of positive reinforcement for it; people are impressed at how well you understand something, at how much you know, at how quickly you pick it up. But even if you are better than average at picking things up and figuring things out, you will eventually hit the limits of what you can do through brilliance alone; there are some things that will just take long hard work no matter what, and then all of that reinforcement for brilliance comes back to bite you because you're not getting the same kind of reinforcement, and you can easily get stuck and lose faith in your self-image.