Deactivate the real bomb - the dining hall could be filled with tons of black balloons with secret messages inside. The kids pop them to find a specific message. I did something like this to start Nicholas's treasure hunt (balloons were canon balls, not bombs).
8 Awesome DIY Secret Codes for Kids - Type your own Top Secret message into the template and it will automatically encrypt to match the Cipher! Great as Spy Party games or activities, for playdates or even in the school classroom as code breaking games $6.50 via SIMONEmadeit.com