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!
