"Irony - a dryly humorous or lightly sarcastic figure of speech in which the literal meaning of a word or statement is the opposite of that intended. In literature, it is the technique of indicating an intention or attitude opposed to what is actually stated."
"Good," said Daru. "And where are you headed?"
Balducci withdrew his mustache from the tea. "Here, Son."
"Odd pupils! And you're spending the night?"
In this exchange Daru is saying that the Policeman and the prisoner are "odd pupils" because he is a teacher and they are in a schoolhouse not a prison. He is indicating that it is odd that the police would be headed to the school instead of a police station, thus the verbal irony.