A flawless system of truancy detection.
Missing a class would involve its teacher writing your name and details of your crime on a yellow piece of paper.
(Lemon Slips sounded infinitely more menacing than yellow paper, though - both effete and mysterious, you can imagine the shudders running down a gentleman's spine.)
These lemon slips were sent to the school secretary, and they were then inserted in the register each morning.
Registers were then left completely unguarded in every form room for a 15 minute period ahead of registration, every day.
There was very little reported truancy at my school.