A loud, idiotic event that is held to make members of a school team feel special about themselves when in truth they are no more important than any other student. They are an attempt at forcibly getting sane students (who are considered emos) to show school spirit, and are only enjoyed by idiotic students, which is generally most of the school.

'The pep rally is a place for pseudo-prostitutes to provoke men into a sexual frenzy which, when thwarted, results in pointless athletic competition.'
- Sunnydale High Newspaper, BtVS (Season 3: 'Earshot')

The feeling you get after you leave a pep rally, after having so much fun and being so hyped up.. to sit there on the boring bus and ponder life realizing that all that fun is over. and you're on the bus.

Pep rally can be used to describe that feeling in any number of situations where things generally suck after being hyped up,