During the last part of the game, Russell is a walking Crowning Moment of Awesome. First, he takes off on a stolen police motorcycle and drives it straight through a barrier into the Townie hideout. A bit later, he says "Russell... likes to HURT people... for PEACE." He then rams the gates to the school campus open with his head (commenting that it "kinda hurt") and he finishes it up by scaring the hell out of the Prefects and chasing them away.
Jimmy and Zoe trapping Mr. Burton in a port-a-potty and pushing it down the hill with a riding mower.
Considering that Mr. Burton has made Jimmy steal panties from the underage girls in the dorms, got Zoe kicked out of school when she tried to report him for sexual harassment, and generally is a dick it's so fitting. At the very end, Jimmy tells Principal Crabblesnitch about Mr. Burton's sexual harassment of Zoe, getting him fired as well.
Chapter 3 had this awesome Gang War between the Greasers and Preppies, on bicycles, with fire crackers.
Jimmy vs. Gary on the roof. It has to be played to be truly savored.
Jimmy getting Mr Hattrick fired for selling tests to students.
Hell, just kick back and watch a random fight happen sometime. If one of the opponents are bigger than Jimmy, they can usually pull off incredible moves that Jimmy could never hope to do in this game.
Nothing is more satisfying than taking down the prefects. These guys will find any excuse to pursue you; they'll give chase if you stay out too late, skip class (which makes things even harder if you do a mission during class time), heck, you can't even toss a snowball without getting in trouble for "violence". Fortunately, not even these guys are immune to a little punishment. You can stuff prefects into lockers, give 'em a swirly, toss 'em into a trash can, pop 'em with your spud gun and rocket launcher, pummel 'em with the Super Spud Gun, tackle 'em, bash their family jewels, and if you're extremely lucky, you could give 'em a No-Holds-Barred Beatdown (all you need is a good strategy). You could also take down the cops, although they're much stronger (it's best to hop onto the roof of a car and attack them with your projectile weapons).
Once you level up your social skills through English class, you could actually gain the ability to scare away the adult townspeople through verbal insults. Just insult them one or two times, give them a shove, and in a matter of seconds, they'll be running for the hills. That's right, a plucky 15-year-old kid could scare off people that are twice his age.
The duel with Edgar Munsen. To put it simply, Jimmy and Edgar clash pipes in a caged arena surrounded by toxic waste.
The last two missions in Chapter 5. Disguised as the school mascot, you and the Nerds sabotage the football team with deliciously-cruel pranks such as rigging a football with explosives, urinating in their drink cooler, and of course, hacking the message on their scoreboard to say "JOCKS PLAY WITH THEIR BALLS". Then there's the battle against Ted Thompson and the entire football team, which is basically a Battle in the Rain on the football field (during the big game, mind you) where you have to pummel the Jocks with your exploding footballs. Taking down Bullworth's toughest clique in front of the entire school is quite an extraordinary victory indeed.
The Chapter 5 mission "Defender of the Castle" has you defending the Nerds' observatory from the invading Jocks. Armed with a spud gun, you charge towards the invaders alongside the armed Nerds, resulting in a massive battle at the barricade. Afterwards, you have to gun down the oncoming forces with the Super Spud Gun.