I wonder if the incident of which you caught the tail-end might have been extended by a sift of suspects to eliminate the innocent. It strikes me as unusual for that sort of trouble to be taken, so it could have been a more mundane explanation, maybe that some of the guilty parties were on the games field and therefore off the premises that afternoon-Coulsden’s near enough my neck of the woods & most schools even then had distant playing fields in London and its’ suburbs. We had 6 rugby pitches at MGS, but often overflowed onto the public pitches in nearby Mote Park for games as well.
The Head might also have had earlier appointments he couldn’t interrupt, of course. Maybe, just maybe, he needed a quick sustainer from the hip flask before battle commenced!