In "Little Ellis and the Boil," Dahl relates the time he was put into the sick room at St. Peter's. Beside him was a seven-year-old boy named Ellis who had a large purple boil on his inner thigh.
This assignment asks you to write one sentence and to then explain it. It seems that your teacher wants you to infer something about the story, which means that you interpret the story in some way and then explain what led you to that conclusion. Here are some ideas for you to consider:
Although he seemed cruel, the doctor performed his duties well.
In today's world, we would not be pleased with a doctor who threw a towel over our heads and then plunged into our leg with a scalpel. But this isn't a modern story; it reflects the medical world of the 1920s. The doctor simply didn't have the technologies or medicines for his use which we have today, and he recognized that leaving this festering boil to grow could put the child at risk of a dangerous infection. Unfortunately, this growth had to be removed, and the doctor did the best he could with the medical tools he could use in the 1920s.
Based on this context, you might also write a sentence along these lines:
The pain Ellis endures in this story demonstrates the way medical treatment has improved in the past century.
In the conclusion, Dahl explains that he thought the doctor was "clever" in his tactics. In this historical context, people were simply expected to "endure" pain because there wasn't much that could be done about it. Even dentists never "bothered" with painkillers and simply performed needed extractions without any anesthetics at all. In today's world, a young child needing a large growth removed from his inner thigh would be given painkillers before and after the procedure. Likely a child life specialist would explain the procedure in child-friendly language, and parents would almost certainly be present to comfort the child. Little Ellis's experience demonstrates how much medicine has changed over the past hundred years, even for the most simple procedures.
I hope this gives you some ideas to consider as you write your own sentence. Good luck!