My school didin't really have groups like that. We all knew each other (graduating class of 360). I think the reasoning was several years before I was in high school there was a definite seperation which caused a fight to break out at an event leaving one teenager brain dead.
I never witnessed this. I am sure there were occurrences in locker rooms, but I minded my own business and never really saw this stereotype go down.
Yes, they are called "Pep Rallies" and it's to get everyone geared up for the big game. We had monthly assemblies that turned to quarterly assemblies with a couple of pep rallies peppered in. They were fun... I miss them!