Head to the West down the hallway toward the red haired girl who is giggling at the doorway.
Head on through the door and go over to the left. Run through the office and go through the back left door after the girl. Head across the next room and through the door in the back left again. Going through this door gets Arno to crash into someone.
Go on through the door in the back right of the room. The girl will be running straight through it as well. Go to the right then left as she leads you out to a small courtyard. Run to the middle of it and approach the girl. She has a little challenge for you.