u can do this 2 ways, 1st way is to open Attitude.dat then go and look for preppy's, here u see # Preppy's
Dispassionate, Dispassionate, Dispassionate, Averse, Averse, Adore, Dispassionate, Dispassionate, Dispassionate, Dispassionate, Dispassionate, Dispassionate, Dispassionate
now change to # Preppy's
Dispassionate, Dispassionate, Dispassionate, Averse, Averse, Abhor, Dispassionate, Dispassionate, Dispassionate, Dispassionate, Dispassionate, Dispassionate, Dispassionate
wat this does is making them fight whenever preppies see each other.
the 2nd way is by lua, this is much more easier for lua modders. just use this code: PedSetTypeToTypeAttitude(5,5,0) this does the same thing but by lua