In my time we just wrote very small on scraps of paper or tore pictures from magazines. We folded them tight into a compact rectangle, then we folded that rectangle across a rubber band between our thumb and index finger and with the other hand we stretched that rubber band enough so that when released, it would propel the folded rectangle of paper across the intervening distance rapidly enough that the adversarial player in the room could not detect the source or recipient of the rectangle without querying the entire congregation.
This method eliminated potentially adversarial middlemen in transit who might, if you chose to pass it through multiple players (servers if you will) - read it in transit though the message was not intended for them, and then use the contents against you later.
It had the disadvantage that one needed to insure that the sender and recipient were in sync in case the aim was off and the message bounced to an unintended recipient.
I once had the misfortune of sending a tightly folded, secure message that was part of a war game being played during English class, and having that poorly aimed message hit the largest mass of muscle in the class right squarely in the ear because the recipient was busy gloating over the success of their previous move and wasn't able to secure the reply in transit.
We all heard the light snapping sound of the rubber band followed by an uncharacteristically loud profanity from the unintended recipient, my own barely stifled gasp of horror, lots of giggles and laughter from the audience, and as they turned - the beginning of the next round of the Inquisition by the adversarial instructor who mistakenly thought we were all watching the English lesson on the board in real time instead of conducting paper war games in the background.
