Sleep is instrumental in forming long term memories IIRC. So it's not at all surprising that better sleep quality would result in improved grades, seeing how they largely depend on you being able to understand and remember stuff for the tests, and other academic activities.

Though I'm surprised that physical exercise did not correlate. Is positive effect on mental performance is well established in the literature. I suppose "more studies are needed" (which means "we don't know" in academia-speak).