They needed to keep Peter's mother from being aware the prank happened. In order to do this they set up multiple planned events throughout her daily routine. This was supposed to divert her attention while the prank was set in motion.
While all of this was happening a rumor was passed around, specifically for Danvers. If all of this went perfectly, then then there would be two secretaries going through the front gate of Bullworth Academy.
The plan was to get Peter's mother to put in her own resignation. To do this they went to a teacher....Galloway. He's been trying to dump his old bottles of booze in some way. Jimmy offered to help him, and Galloway gave Jimmy the entire stockpile. Every time Peter's mother had her attention diverted, they would have more time to set up the prank.
As they did this A rumor was set out stating that Crabblesnitch would meet any demands of Danver's for her to return.