Here's my thoughts.
Chapter 1 - Pete didn't interfere in the fight setup as he might not have known it was going to be Jimmy - and the game very much indicates that fights are relatively common there.
Chapter 2 - Gary was pandering to the egos of preppies - and since Jimmy was essentially the "poor kid from out of town" it made it easier for Gary to make up random rumours.
Chapter 3 - The Greasers asked Jimmy to help Johnny after he'd already helped out other people. Jimmy refused rather rudely, setting things up for a confrontation. Naturally it wasn't helped by Jimmy trying his luck with Lola (along with half of Bullworth Academy)
Chapter 4 - In regards to the Jocks, they were clearly antagonistic to Jimmy & Pete at the start of the chapter. As for the Nerds, Jimmy had wanted them to help him, but they were rather rude in their refusal to help (much like Jimmy's rude refusal to help Johnny caused aggro in the previous chapter).
Townies - Well, they very much hate everyone in Bullworth Academy - and were responsible for starting the fire in the Gym.
Final Chapter - Gary had kept quiet, staying "behind the scenes" and stirring up the resentment the various clique leaders would've had for Jimmy as each and every one of them had lost in a fight with Jimmy. Once the leaders were against Jimmy, the rest of the cliques followed.
