Undoubtedly, some of your attendees will arrive intending to spend money at the auction but will leave the event empty-handed with their wallets full. Give them a chance to spend that money.
Have a table set up as they leave where they can sponsor a classroom or contribute to a special project. Make sure they receive a personalized thank you card for any contributions that are made or even a small token of thanks. That way, they will be sure and check the table the next year when they leave, even if they are not walking away empty handed!