A person in high school or junior high who thinks he is good at a sport because he beat all the un-athletic people in gym class.
kid 1: (shoots ball and gets it in) "dude I deserve a varsity jacket for smoking all your asses in basketball."
kid 2: "Man, stop being a gym class hero and play the real sport."
All kids in unison: "GYM CLASS HERO! GYM CLASS HERO!"