More the threat of punishment this one. I was at Alleyn's in the 90's. Occasionally we used to get kids from William Penn (as it was then) invading the playing fields. This happened one Thursday which was CCF day and the School Serjeant, fed up with these uninvited visits, grabbed 4 sixth-form army cadets all in their combat jackets and boots, issued them with an SA80 each from the school armoury and marched out across the playing fields towards them. Unsurprisingly, the visitors decided they no longer wanted to be on site and legged it but the Serjeant simply yelled "YOU BOYS - STOP RIGHT THERE!" Maybe it was the fear of getting shot but they stopped, he summoned them over and, surrounded by his armed guards, delivered the mother of all bollockings to them. We never had any problems with William Penn after that!