Answer:
Presently it occurred to him that he wished he was sick, then he could stay home from school.
Explanation:
The above sentence actually shows that Tom is to be resourceful.
We discover from the passage that Tom wasn't feeling like going to school that Monday. So, he was looking for a way to make an excuse for not going. He started thinking and cleverly looking for ways to avoid school.
It is then he wished he was sick, so that he could stay home from school.
If you want to explain to a foreigner how to learn when to use these words, you can say that the go from more general to more specific (in: more general, at: more specific).
in: for objects inside other objects, or for location in a big context: year, country,
on: on top of other objects, or for location in middle-scale contexts: day, street
at: for location on a very small scale: moment (exact time), at specific address<span />
C. gathering and analysis of information
If you read the book you will understand! :D