By this I mean you can speak to the characters as you would do with other students. I have provided a very short walkthrough on how to meet some characters without mods.
PETEY
Petey is probably the easiest character to meet. He often appears on school campus at the begining of chapters. You can greet him at the begining of Chapters: 3,4 & 5.
You can also talk to him just before starting Boxing Challenge Chapter 2, where he is sitting on the pier. He will somehow disappear after the mission has started.
WILL HE FIGHT: Well, I found that Petey seems more agressive than other non-cliques with everytime me provoking him, 6/10 he'll fight back.
TED THOMPSON
Only one way to meet him in free-roam and that is Chapter 1 in the mission "Slingshot" where instead of knocking out the jocks, jump down.
After the mission has been failed, you can greet Ted. His shirt number is 0.
WILL HE FIGHT: Yes. He will. But because of Slingshot being a fairly easy mission, he'll be KO'd in 2-3 punches.
GARY SMITH
Loads of ways to meet him in free roam.
You can greet him in "Welcome to Bullworth" after knocking Wade out. For some reason, he will not speak but will fight if provoked.
In the PS2 version of Bully, there was a glitch which caused Gary to spawn in Blue Skies Industrial Park or on Bullworth Campus in Chapter 6. Quite rare though. The Scholarship Edition fixed this bug.
Start the mission "That Bitch" without stink bombs. After the cutscene has finished, you'll be greeted with another cutscene where Gary told Jimmy about Fatty having stink bombs. After that Gary can be interacted with normally and will fight if provoked but will run away if pushed or bullied.
After completing Halloween, Gary & Petey will be at Jimmy's side but cannot be interacted with.
After completing "The Big Prank" Gary will be in the school but the odds are heavy he'll run into a class room since Mr. Burton will go straight for Jimmy or Gary.
A glitch which has happened to me 2 times in Final Showdown stop Gary from going up the ladders. He'll just continue insulting Jimmy, but if assulted you'll have to restart. He can be interacted with, but he'll ignore you and just stand there. If you grapple him without harming him (chucking or ppucnhing etc.) he'll break free and say "I thought we were buddies you maniac!"
WILL HE FIGHT: In most of them, yes. But for the "That Bitch" one, if you shove him he'll run away.
Johnny Vincent
Only way I know how is to start Jealous Johnny and just talk to him. You can get him to fight you by letting him see you harming another Greaser, but remember, if you even touch him, mission failed.
WILL HE FIGHT: Only if you insult him enough or if you let him see you attack another Greaser.
Derby Harrington
I have no idea yet on how to meet him in free-roam.
Edgar Munsen
After rescuing Russell from Wonder Meats and the cutscene with Edgar and Jimmy outside Bullworth, just stop and talk to Edgar. However, he'll act like he has 55% respect since if you greet him, he'll greet back but if you insult him, he'll insult back and one hit is all it takes to get him to fight you.
He has infinate stamina.
WILL HE FIGHT: Yes. He has infinite stamina.
Russell Northrop
Easy as hell. Sometimes he'll spawn in the carpack or outside Bullworth's gates. In Chapter 1, he'll fight easily. But in Chapter 2,3,4,5,6 you'll need to hit him 2 times.
Earnest Jones
After finishing Paparazzi, he'll walk into the library and come back out a few moments later. He will fight if provoked but will stop fighting after a few seconds for some reason. He cannot be humiliated.
WILL HE FIGHT: Yes if provoked, but will stop fighting after 10-15 seconds.
Well that's it for now. I'll be working on a video which shows how to meet gang leaders in free-roam with no mods soon.