Lydia,
I envision this lady teaching a class with her back to the students as she writes on the blackboard (whiteboard - whichever). This lady is wearing a pair of poly-something, form-fitting pants and as she writes on the board, her butt starts to violently twitch (your description on the twitching). The class begins to snicker as the teacher's backside goes into its own, rhythmic dance, cheek to cheek!
The teacher ignores the laughter and continues to write but its no use; the class goes from snicker into roaring hilarity. With her back still to the class, the butt dance finishes in a diminished tempo of twitches and the class is never normal again.
From that point on, each time the teacher turns her back to write on the board, students gaze with bated anticipation at the now infamous twin cheeks that have now been individually named by the students.
I'm up for grabs on the names, folks.