We had a similar system at my school, but it was like Russian roulette.
We knew that we would only be punished if the headmaster actually came out of his room and saw us there.
If he did, it was a question of whether he was in a good mood, or in a hurry or not.
If both, we could escape with just a telling off on the spot.
If not, he could pile on the tension with a Long lecture, still not giving any clues as to what he was going to do.
At worst, at the end of the lecture, he would reach into his low desk drawer (stomach tightens and heartbeat quickens) and pull out that old slipper of his.
The whole horrid system made waiting so much scarier.
If we had known that it would always be a quick 6-of-the-best whacking with the slipper, we probably would have resigned to our fate.