There is nothing - nothing - worse than getting caught up in somebody else's in-joke. Apart, perhaps, from coming home to find Yoko Ono wiping her arse on your curtains. But getting caught up in an in-joke is nearly as bad.
And school teachers are the worst:
"Here, Coleman", said Mr Wilko hardly able to hide his mirth, "Take this message to Dr Jenkins next door."
"What message, sir?"
"This: [posh voice] 'Oh, Hello!'"
[rolls eyes] "What? Again? That's the third time today sir."
There is a knock on the door.
In walks Simon from the class next door. He looks nervous.
"Message from Dr Jenkins, sir."
"It's... It's... [posh voice] Oh, hello!"
A roar of childish laughter can be heard from the classroom down the corridor.
"You still here, Coleman?"
And they dare to call ME mad.