Bully Cheats

Black Skate Shoes: Travel at least 50.00km on foot.
BMX Champion: Complete one of the bike races.
Boxing Outfit: Beat either 'Boxing Challenge' or 'Prep Challenge' in Chapter 2.
Crash Helmet: Place 1st in the Kart Race at the Carnival
Dunce Hat: Go to the boys locker room in the gym and turn on all the showers.
Edna Mask: Destroy 19/19 Tombstones during Halloween.
Fast Food: Complete the Burger Joint errand found in Bullworth Vale.
Firefighter's Helmet: Pull the Fire Alarm 20 times.
Gnome Costume: Smash all garden gnomes.
Gold Suit: Buy all clothing in the game.
Green Ninja Outfit: Hit stuff with projectiles (eggs, etc.) 1000 times.
Grotto Master: Collect all G&G cards.
Jimmy's Skeleton Halloween costume: Beating the mission 'The Candidate' will open up the mission 'Halloween' which unlocks the costume.
Orderly Uniform: Complete 'Finding Johnny Vincent' in Chapter 5.
Pirate Hat: Beat up pirate on island near Beach House.
Prison Uniform: Beat all the detention mini games at least once.
Red Ninja Outfit: During Halloween (must be done during Halloween) at Bullworth Academy, complete the Big Prank task.
School Mascot: Beat 'Nice Outfit' in Chapter 4.
Tiny Swimsuit: At the Beach House location, find the Preppie with a blue mission on the beach and beat his swimming time.
Wrestling Uniform: Beat Gym 1.

Unlockable Girlfriends
Completing the following missions will unlock a certain girl to be your girlfriend (enabling you to kiss them, thus receiving a health bonus)

Beatrice as a girlfriend: Complete the mission 'That Bitch' in Chapter I
Lola as a girlfriend: Complete the mission 'The Tenements' in Chapter III
Mandy as a girlfriend: Complete the mission 'Discretion Assured' in Chapter IV
Pinky as a girlfriend: Complete the mission 'Carnival Date' in Chapter II
Zoe as a girlfriend: Complete the mission 'Smash It Up' in Chapter V

Weapons
You gain new Weapons when you complete certain Tasks.See it Below:

Bottle Rocket Launcher: Complete the Nerds Challenge
Camera: Complete the Task "Hattrick vs Galloway"
Digital Camera: Complete the Class Photography 4
Do FireCrackers in Your Chemical Kit: Complete the Class Chemistry 1
Do Itching Powder in Your Chemical Kit: Complete the Class Chemistry 3
Do Stink Bombs in Your Chemical Kit: Complete the Class Chemistry 2
Eggs In Your Room: Complete the Task "The Eggs"
Skate: Complete the Task "Defend Bucky"
Slingshot: Complete the Task "The Setup"
Spray Can: Complete the Task "Tagging"
SpudGun: Complete the Task "StrongHold Assault"
Super Slingshot: Complete the Task "The Candidate"

Glitch: Dodgeball Invincibility
During dodgeball, press [L2] to bring up the help screen right before an opposing player has the ball. While the help screen is up, your players will be not be knocked out if they get hit. The ball should bounce harmlessly off your player, allowing you to pick it up and go on the offensive.

Easy Defeat of Prefects/Cops
There is a glitch within Bully that allows you to hit a Cop or Prefect without them being able to get to you.

There is a wooden bridge across the entrance to Bullworth School. If you jump onto the hand rails of the bridge while the authorities are after you and shoot at them from there they can never get you. They try to grab but can't get you due to the glitch.