Instructions
Travel to the right side of the school hall, directly opposite the troll. Notice a trunk that rattles directly below a portrait.
Open the trunk and use "Riddikulus" on the Boggart that pops up. He will drop a key.
Use your wand to pick up the key and carry it to the far-right corner of the room. Find a locked door. Use the key on the lock to break through the bricks and access the door to the garden.