Dropping me down to a B

Why makes this Joke funny?

The humor in the joke:

"I was furious at my English teacher for dropping me down to a B for missing just a single period..."

is due to its play on the double meanings of the word "period" and the unexpected twist at the end. Initially, the joke seems to be about academic grading and punctuation:

1. The first meaning of "period" refers to the punctuation mark used at the end of sentences, suggesting that the student was penalized academically for a minor punctuation error.

However, the punchline introduces a second, unexpected meaning:

2. The second meaning of "period" refers to the menstrual cycle, where missing multiple periods could imply pregnancy. This introduces a sudden and surprising shift in context from an academic concern to a personal one.

The humor arises from the ambiguity and the clever twist in the narrative. The joke plays on the expectations set up by the first part and delivers an unexpected punchline that completely changes the interpretation of the "missed periods." It also plays on the double entendre of "worried enough to increase it to an A," suggesting that the teacher might be anxious about the potential implications of the student`s missed periods and might change the grade out of concern or nervousness.