The parenthesis denote the action that goes with the line:
Clap you hands (stomp feet)
Stomp your feet (clap hands)
We’re the __________ who can’t be beat
We’re number 1 (hold up two fingers)
Peace! (hold up one finger)
The ______ has been filled in many ways, from the inappropriate (“retards” and replacing the line with “special ed is really neat”) to the harmless (“Morons”) to the topical (“insert name of politician, school team, etc, here”). Dates back to at least the early 90s, probably further. I heard it in about 91, and when I moved to Georgia a few years later, the kids there already knew it.