In Buffy the Vampire Slayer's third season finale, we get this dialogue from our heroine:
Buffy: I wish I could be a lot of things for you. A great student, a star athlete ... remotely normal.
I'm sure we can all sympathise with Buffy's wish to please her mother, but it makes me wonder...why can't she be a star athlete?
As the Slayer, Buffy is "naturally buff", with strength, agility and reflexes naturally greater than average with little need for her to work out. These attributes would surely be of enormous use to an athlete.
So why didn't she get involved in school athletics?
We saw several times that Principal Snyder gave extensive privileges and excuses to his star athletes. See the swim team getting away with sexual assault in Go Fish, and Snyder coercing Willow into doing Percy's work for him.
We also had several incidents showing the high social status of athletes at Sunnydale High.
Obviously, out of universe Buffy was supposed to be a social outcast struggling to balance school and slaying, but is there a good in-universe reason that she wasn't involved in school athletics?
I don't buy that it would have interfered with her Slaying too much. In fact, it would probably have improved her slaying, with 'practice sessions' providing an excellent cover for slaying duties, and extra support from the faculty reducing her studying stress.
Imagine, for example, if she had taken up martial arts - suddenly not only is she permitted to take time to train in combat, her tendency to respond to situations with violence becomes more understandable and excusable.