There are various setups to this punchline. Here's one:
One day Johnny walked into the classroom, and the
teacher said, 'Johnny, do you have your report done?' He replied, 'no ma'am.' She said, 'if you don't have it done by tomorrow then I'
m going to make a call to your
parents.'
As Johnny is walking home from school he looks to his left and sees two greyhounds racing,and one gets so far ahead of the other one that it just stops and the other one rams its head right up its a**.
Johnny takes out a piece of paper and writes it all down, saying to himself, 'This is going to be my report.'
The next day at school the
teacher says, 'Johnny, do you have your report done?' He says, 'sure do.' So he goes up to the front of the class and starts telling them what he saw. 'Yesterday I was walking home from school when I saw these two greyhounds racing, and one rammed its head right up the other's a**.' The
teacher says, 'Johnny, we don't use the word 'a**' in the classroom, it's rectum.' Johnny said, "Rectum? Damn near killed 'Em."
Or this shorter version:
Teacher: So Johnny, how was your weekend?
Johnny: Horrible. A car hit my
dog, right in the a**!
Teacher (correcting Johnny's language): Rectum.
Johnny: Wrecked 'em? Damn near killed 'em!
