I read it in senior high English. Until I took the class, it was one of my favorite books. After dissecting every fucking word, I have yet to pick it up again.
I don't even remember the n-word being discussed or not discussed. Our school was small and not divided on racial lines, but rather on social and economic lines (athlete versus non, haves versus have nots).
You can't un-ring the bell. Ignoring the past won't make it go away. Lots of "classic" books out there reflect the prejudices of their times - but they are not vilified because of one word that was commonplace and accepted at the time the book was written.
You don't have to make the word "black" the focus of the entire lesson either - lots of other issues to delve into - physical abuse, poverty, rivers as the source of all commerce, river towns the source of vice and corruption, superstition guiding actions (the spider in the candle), the hypocrisy of self-righteous do-goodies that try and force religion and "morals" down another person's throat while they themselves owned slaves, the lack of equal justice for all - justice only for those white and wealthy - neither Huck nor Jim would have received a fair trial if they had been caught stealing... it was not all about race, but as much about the class system as anything.
We've come a long way, baby. Teach it.