Sending secret hidden messages.
Put your message...
on a sticky note under a taped down poster
folded into a tiny little square and put on the area of the chalk board were the teacher puts the chalk. The receiver can pick it up when he sharpens a pencil of lines up.
To make portable hiding places, you could...
roll it around the ink cartage in your pen and put it back together. Next have the receiver say he lost his pen and you give him your pen/message.
Some books have space between the cloth of the spine and the glue that holds the pages together. Hide it there. It must be a library book or it doesn't work. Tell your receiver to get the message in the school library book bla, bla, bla.
Make sure both of you (the sender and the receiver of the message) know when, were, and how the message is passed.
You should use signals such as if there is a X on the chalk board after 9:00 the message is under the drinking fountain. If there is a O on the chalk board after 9:00, the message is under the teachers desk.