Head teacher talking about recent vandalism during school assembly:
"And to those of you who wrote Mr. Smith's telephone number on the door of the girl's toilets, he would like to make it clear that the last digit is a 7 and not a 4."
Alright, listen up, you simpleton! Basically, some hooligans wrote Mr. Smith's number in the girl's restroom, implying he's, uh, "popular" with the ladies. The head teacher's announcement is a funny way of clarifying the number while also hinting at the prank.