Here are a few sentences from the text which have idiomatic expressions. Can you say what each means? (You might want to consult a dictionary first.)
Our entire class is quaking in its boots.
Quaking in its boots means shaking with fear or extreme nervousness. It indicates that the entire class was terrified about what was to come.
Until then, we keep telling each other not to lose heart.
Not to lose heart means not to lose hope, courage, or confidence. It implies encouraging each other to stay positive in a difficult situation.
Mr Keesing was annoyed with me for ages because I talked so much.
For ages means for a very long time. It is an idiomatic expression used to indicate that Mr. Keesing's annoyance lasted for a prolonged period.
Mr Keesing was trying to play a joke on me with this ridiculous subject, but I'd make sure the joke was on him.
The joke was on him means the joke backfired on the person who initiated it. Instead of making fun of the speaker, Mr. Keesing himself became the target or butt of the joke.
Explanation
The textbook context explains that idioms are groups of words with a fixed order and a particular meaning that is different from the literal meanings of their individual words. Therefore, these expressions cannot be understood literally. 'Quaking in its boots' refers to trembling with fear rather than literal boots shaking. 'Not to lose heart' means retaining hope, not physically losing a heart. 'For ages' idiomatically denotes a long duration rather than literal epochs. Finally, 'the joke was on him' signifies a situation where a prank rebounds on the prankster, making them the victim of their own joke.