Instructions
Go through the tutorials and develop your character to level six. You should have at least one spirit and have dueled several characters.
Equip the first spirit you earn and then go to sleep in your dorm. At school the next day, you will hear about a "school duel" that will take place after hours.
Gather all of the spirits you can find in the school hallways. You can take as much time as you want, as the school duel is a fixed event that will occur no matter when you exit the school.
Go through the entrance of the school and proceed to the "duel field" that appears on your map. Once you enter, you will participate in your first school duel (you don't get an option to back out.) Once you emerge victorious, you will unlock the school duel feature and can return to the field at any point in the game (after school hours) to develop your characters and use your card deck.