It was a robust cardboard box in which a consignment of books had been delivered to the school. (For the benefit of younger teachers who might be puzzled by this, I should explain that there was a time when sets of new books occasionally arrived at schools.) Bryant and May commandeered the box and used their crayons to adorn it with a volume control, contrast, vertical hold (ah, those were the days) and a channel selector.
They scissored out the shape of a screen, clambered into the box and immediately commenced to broadcast. I think it is fair to say that in 1974 they invented day-time television: banal, repetitive and cheap. In the absence of cameras, they used their elbows to ensure that they enjoyed equal screen-time, or sometimes settled on having both their profiles face-to-face, as Mel and Griff were to popularise many years later.
They perfected the TV interview, becoming disturbingly adept at the ingratiating smile, the innocuous exchange and amusing anecdote. Although nothing would induce them to open their mouths during hymns in assembly, once inside their box, they would happily re-enact Top of the Pops - yes, younger teachers, the programme is that old. Bryant would thrash his air guitar, May would mumble the latest from Bay City Rollers. Bryant, who fancied himself as another Jimmy Tarbuck, would also tell jokes. Well, it wasn't so much tell as read them from the Life's Like That pages of the Reader's Digest which May used to hold up for him. In fact, it's probably true to say that Bryant and May invented the autocue.
They did outside broadcasts. During games lessons, Bryant and May claimed press accreditation. For the whole of the match, Bryant would pretend to be a cameraman. He'd race up and down the line, his eye to a non-existent viewfinder and his right hand furiously pretending to wind the film - particularly curious, since that sort of camera had become obsolete long before D W Griffith shot The Birth of a Nation. Even more galling for the games teacher, May provided a running commentary. You try teaching the basics of ball control against a background of high-pitched pre-pubescent prattle. Needless to say, on more than one occasion, they were sent for an early shower where they enjoyed the best of the hot water and - no doubt - interviewed each other on the mixed blessing of not being quite like everybody else.
In less enlightened times, Bryant and May would have been marched off to the juvenile wing of Bedlam. But - with the exception of the games teacher - the staff tolerated them.
Indeed, I even grew to depend upon them. In those unforgiving minutes, when my lesson had ended but the bell had not yet rung, I knew that one well-aimed nod would have them scampering into their box. A reluctant hush would descend over the class, and the tenacious but talentless twosome would embark on another of their moronic marathons.
It seemed in the Seventies that they were a hopeless pair. But digital television, with its promise of countless channel and endless hours of air time to fill, should change all that. If Bryant and May haven't appeared on your screen yet, they soon will.