The ambiguity of extra 

Ask a random American to name some of the extracurricular activities commonly offered by the nation’s public schools, and they’ll rattle them off easily enough: the jazz band, gymnastics, baseball, yearbook, Future Farmers of America, the debate team, the robotics club, and on and on. But ask them why these particular activities are offered as extracurriculars, as opposed to regular classes, and they’ll likely respond with a blank stare. 

Sure, we all know that the rules are different for extracurricular activities — you can choose whether to participate, and you don’t have to worry about tests and grades. But that doesn’t answer the question: What defines them as extracurriculars in the first place? By what logic do we assign, say, history and chemistry to the regular curriculum, while categorizing debate and robotics as extra?