Alternate costumes: just do the following:
Fast food costume: finish the Burger Joint errand in Bullworth Vale.
Firefighter's helmet: pull the fire alarm 20 times.
Gnome costume: smash all the garden gnomes.
Gold suit: buy all the clothing.
Black ninja costume: completely fill your yearbook.
Black skate shoes: walk 50 km.
BMX champion costume: finish a bike race.
Boxing costume: finish the Boxing Challenge or Prep Challenge in Chapter 2.
Red ninja costume: finish the Big Prank task during Halloween at Bullworth Academy.
Orderly uniform: finish "Finding Johnny Vincent" in Chapter 5.
Pirate hat: beat up the pirate on the island near the beach house.
Prison uniform: finish all the detention mini-games.
Pumpkinhead mask: destroy all 27 pumpkins around the school and inside the main building during the "Halloween" event.
Wrestling uniform: finish Gym 1.
Dunce hat: turn on all the showers in the boy's locker room in the gym.
Edna mask: destroy 19 tombstones during Halloween.
Green ninja costume: hit targets with projectiles 1,000 times.
Grotto Master costume: get all G&G cards.
School mascot costume: finish "Nice Outfit" in Chapter 4.