Since I have read Huck Finn in school, I am not at all clear why it is necessary for a teacher to drop the n-bomb in order to prepare students to understand the text.
As for the book itself it's a classic.
It sounds to me as if the teacher failed to properly incorporate step 6 of the learning plan: [Step 6: Decide as a class how to handle the word n----- when it appears in the book. Skip it? Replace it? Say it? Should each person have a choice of what to do?]
The student's objection to the word could have been used to springboard a class discussion on this part of the learning plan and different ways people can respond in life to words or ideas that they don't like. Instead, the teacher apparently simply continued to use the word even though it was clear there was a problem.