Student athletes are better than average students, despite having some sports severely dragging down this record. My impression is that these students know that they have less time to study and thus do not fool around like the others. They develop superior time management skills and thus obtain better results with less time to study.
Another aspect I did not think about is that athlete are more competitive. It is in part selected and in part learned: they get into sports because they are competitive, and they become competitive because they are athletes. Jérémy Celse shows that this is a mixed blessing, though. Once they have graduated, they also get an edge from being competitive and used to compete. However, athletes are less cooperative and find satisfaction in hurting others. Using an experimental approach, Celse finds that (self-declared) athletes behave anti-socially and with more jealousy than others when learning they did worse than their experiment partners (or opponents). In other words, do not count on athletes to be team workers. It is unclear whether this applies also for those practicing team sports. The variable was captured, but I see no result using it.