It seems hardly possible to me that such things should happen without its coming to the ears of the prefects who were the perpetrators of the deeds in question.

Here we have a third example of the same thing.

If neither of you took any actual part in screwing up this door, I am still inclined to think that you must have been cognizant of the act, and I demand to know the names of the offenders.

Take time to think before you answer.

I warn you once more that I am determined to sift the matter to the bottom." Once more the two prefects protested that they had not the remotest idea who had played the trick on Mr.Grice.
Dr.Denson frowned, and sat for some moments without speaking, rapping the blotting-pad in front of him with the butt end of a seal; then remembering the presence of the small boys, he turned towards them with an inquiring look.
"Well ?" Diggory's face wore something of the same expression which Jack and Mugford had seen upon it when long ago their friend first distinguished himself at The Birches by going down the slide on skates.