Answer: to portray Huck’s father as uneducated and ignorant.
In this excerpt, Twain most likely uses humour to highlight the absurdity of the father's sentiment. This is done in order to portray Huck's father as uneducated and ignorant. The father tells Huck that he does not see the importance of school. Moreover, he thinks school is damaging because it will teach Huck to feel superior to his dad, who is unlikely to have attended school.
This is a theme. The difference between those two is that the plot is similar to the storyline, whereas themes are general thoughts being analyzed. In this case, this is a theme, whereas something more detailed with description of events would be a plot.
