Possible Clues:

• Note how it is back in the date and will go off
• Go off assignation outside school
• Set off from college in time
• Blast school when there's a girlfriend around!
• Set off for assignation out of school
• Set off for appointment outside school
• Blow
• Go off, as a bomb
• Set off, in a big way
• Girlfriend holds message upside down? Blast!