Head to the South Park Elementary school. Once inside, defeat the ginger hall monitor. Enter the door to the east and your path will be blockaded by the monitors. Stand in front of the fire and use Dragonshout to knock them out and clear the way.
Continue north and try to open the cafeteria. Mackey will tell you that you need a gold key to open the door. Head to the west and you'll find another hallway monitor waiting. You can skip combat by shooting the telephone to get his attention then knocking over the stack of chairs.
After defeating the hall monitor, head further to the left and you'll find yourself locked out. Set Kenny as your buddy then press Q to give a buddy command to the nearby hallway monitor. After getting past the barricade, enter the janitorial room and loot the items inside.
Continue to the right side and encounter another blockade. Aim for the bulletin board on the wall and wait until two monitors are underneath it. Shoot it to knock them out then engage the kid that has the brass key. After defeating him, backtrack and use the brass key to open the Faculty Office.
Once inside, shoot the ashtray to light up the firecrackers to take out one monitor. Next, stand in front of the fire and use your Dragonshout again to eliminate the guards. Next, shoot the stack of books to take out the last hallway monitor. Approach the injured kid and order Butters to heal him up. You'll get the Silver Key afterward.
Backtrack to the east and use the Silver Key to open the Counselor's room. Loot the items inside and shoot the Gold Key from the shelf. Save your game and head to the cafeteria door. Attempt to open it and you'll have to fight a boss. Defeat him and obtain the Hallway Monitor Key. Open the cafeteria door to finally bust out Craig. He'll added to your friends as well.
Head back to Kupa Keep and talk to Cartman to continue with the story. Head to the training grounds to learn a new magic from him. After successfully completing the training, the quest will be complete and a new one will start.