Final answer:
In the context of the story "Big Squeak and Little Squeak", the word 'fun' is the one that is not to be used in the classroom, signifying a focus on discipline over leisure.
Step-by-step explanation:
The word that is mentioned in the story "Big Squeak and Little Squeak" that should never be used in your classroom is 'fun'. This particular word may be highlighted in the story to emphasize a rule or a moral lesson that contrasts the value of responsibility or discipline over leisure in the educational environment. It's a narrative approach to teaching students about the importance of taking their studies seriously.