I think the general line, even with our agreed lesser use of CP down here, was probably the same both sides of the border when it came to mob-handed misbehaviour-the Claude Rains/Captain Renault approach:
“Round up the usual suspects!”
AND Claude Rains was also born in Camberwell, SE5!
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.