If you have a pair of binary boxes:

The most number of students you can have using a pair of boxes is six.

This gives three students at each end.

• One student is in control selecting the message and each letter to send as well as deciding what to do when replies come back.
• The second is the coder/decoder . who looks up the letter (or control code) to be sent and converts it into the light sequence. When a 6 digit code is received that person also looks up the code and decodes it back into the letter or control message.
• The third person is the operator who sends the sequences given to them and writes down the signals (rrgrgg) received.

This encourages teamwork, though it is important to rotate children around.

Therefore for 1 class of thirty, you need 5 pairs of boxes as a class set

Plus you can then use the breakout box with the pi to verify the comms, and show the children how easy it is to eavesdrop into the conversation.  (or spy on them !!)

Clearly, this leads to the concept of encryption.

The minimum you need is 2 children at each end. Below that and one person is trying to do everything and … cognitive overload as well as impossible time management.