5 ideas
In the sketch of being examined, the teacher slowly runs a finger down the class register while the narrator's mind races through bargains, prayers and calculations about whose name is near his and whether the finger will stop on it. The sketch treats those few seconds as an ordeal that expands into a whole inner drama. It shows that anticipatory dread of an arbitrary, visible selection can outweigh the test itself.
Paired sketches show a good student and a bad student answering at the blackboard. The good student experiences the exam as a smooth performance of knowledge. The bad student experiences it as frantic improvisation: reading the teacher's face, catching whispered prompts, and stretching half-remembered words into sentences. An identical external situation is two entirely different events depending on the resources the person brings into it.
Before showing his bad report card to his father, the boy builds an elaborate argument full of mitigating circumstances, relative comparisons and appeals to injustice. He is trying to reframe the failure before it can be judged. The humor comes from how sophisticated his rhetoric becomes compared with the plain fact it is meant to hide, which shows that the fear of judgment is what trains a person in persuasion.
The adult narrator deliberately re-enters his schoolboy perspective. From there the teacher's desk is a throne, a bad mark is a catastrophe, and a classroom joke is a historic event. Taking a small world at its own inner scale, rather than correcting it with adult proportion, reveals the real emotional stakes that adult hindsight tends to dismiss as trivial.
In the sketch where the whole class breaks into laughter, one small absurdity spreads through the room. Each boy's effort to suppress his laughter makes it worse, because the forbidden setting and the sight of others struggling feed the laughter back into the group.