It is an interesting question.  I think a teacher who wants to teach any “controversial” text or a text that someone may deem inappropriate for a specific educational setting needs to answer two questions about the text.

1. What exactly is their intent in teaching the text?  I mean, what is it about the text that is unique or powerful from a literary perspective? 
2. Is there an acceptable alternative to the offensive text?

If the particular message of an “offensive” work of literature is not overwhelmingly unique and powerful, or extremely relevant to a particular aspect of the curriculum, I think that it is just not worth the effort. 

I love Twain and I love Huckleberry Finn.  However, most of the time it is just not worth it.  Like previous posters have mentioned, it takes a particular educator with a specific set-up and backdrop to pull off teaching this specific book.  A vast majority of the time it is going to be easier and more time effective to simply teach something else, especially if the students are not really losing out by reading the alternative text. 

This is of course completely ignoring negative parent reaction and outside social pressures that teaching this specific book brings.      

I can only imagine how the use of the "N-word" and the treatment of Jim by so many characters might seemingly first be seen as offensive by people of color. I know that when we read it in class, we never read the N-word aloud, not only out of respect for any black students, but because I personally find the word offensive. There are literary purists that believe that within the context of the book, it can be read aloud—but in creating such an offensive climate, I would assert that it is difficult for the true message Twain was sharing to make it through to the individual reader, and the class as a whole.

What I have always found so heartwarming about the book is the "ignorant" backwoods Huck Finn, without formal education or religion, and his ability to find the true meaning of brotherly love and deep, abiding friendship. Huck does trick Jim once into believing that Huck has been lost from the raft, presumably having died. Huck is thoughtless in this, and when he sees Jim's absolute devastation, he is ashamed. From that point on, Huck has learned a lesson about caring for the feelings of others—regardless of race; however, he also learns how much he means to Jim, the runaway slave who has already lost his family.

Twain allows us to study the mind of a Southern born and raised youngster who believes that slavery is an acceptable way of life. We see him struggle with with his conscience in facing what society demands—turning Jim in. And we finally find that he is loving and strong enough to defy society and God (or so he has been taught), to protect Jim and go to hell if that is what it costs him. 

Huck writes a letter to Miss Watson to tell her where Jim (as her runaway slave) is. At first he feels relieved of a great burden, and is sure he will be able to pray. He is amazed "how near I come to being lost and going to hell." But he does not pray and he makes no move to send the letter. He begins to think. And in this we find Huck not to be ignorant at all, but mindful of what he is doing, observant of what society and God expect, and strong enough to make his own choice, despite what the norm of the South is at that time. In making up his mind, he believes he is doomed, but is willing to be so for his best friend:

It was a close place. I took it up, and held it in my hand. I was a-trembling, because I'd got to decide, forever, betwixt two things, and I knowed it. I studied a minute, sort of holding my breath, and then says to myself:

“All right, then, I'll go to hell”—and tore it up.

Huck makes the right choice, ironically, even though he believes he is eternally lost in that moment. I believe there is something heroic in his decision, and this is a rare thing for young people to see in society today. It's not an easy book for many kids to read—not being readers of any kind outside of the classroom—but the overall message is a fine one, and I believe it does belong in today's classroom.

In this Eggshell society, it may be necessary for teachers to prepare some explanatory material before the presentation of The Adventures of Huckleberry Finn.  But, if a teacher or school system shies from this novel that Ernest Hemingway felt was the benchmark of all American novels, literary America is in serious trouble. 

""The best book we''ve had. All American writing comes from that. There was nothing before. There has been nothing as good since."

Why is it all right for songs to be blasted out of cars' speakers in which every other word is the n---word?  Is it only because the drivers are of a different race?  Come on, now. Mark Twain is no racist; in fact, he depicts the lack of common sense in those who call Jim this pejorative term.  And, while Huck at first uses this word for Jim, once he spends time with the man, Huck grows to love and respect him.  Is this not the main theme--a worthy theme, at that?  Why do teachers and parents want to remove this book, then?  There is a valuable lesson in Twain's work, one that students will miss if they do not read it.

Besides, if there is a crusade made against this book because of a pejorative term used, what about the satire of white people with the clannish Grangerfords. Why is there not a protest about this demeaning depiction?  For Mark Twain, realism was the criterion, not racism.

I absolutely believe that Huck Finn should be taught in the classroom.  I teach The Tragedy of Pudd'nhead Wilson to my eighth grade students, and as it was mentioned in several posts above, a teacher absolutely has to be very smart about how she introduces the text.  Despite the age of the book, the difficulty of the dialects at time, the book remains amazingly relevant in terms of the themes inherent in the novel.  To disregard the book purely on the basis of the offensive slang is to miss all of the truly wonderful ideas that the book does present--about what freedom, friendship, and true equality really should look like.  We, as a society, are not so far removed from the Civil Rights movement and even the Civil War that we can pretend slavery and discrimination never occurred or does not occur today.  Huck Finn serves as a genuine eye-opener for many students in high school today.

I think teachers have to be careful with how they introduce this book.  If they have already taught other books with the N word and these themes, that is one thing.  But I strongly believe you do not just hand the kids the book and tell them to go home and read it.  They have to be gently guided through it.  Some kids will see slaves referred to with that word and really be upset.  It can really bring up good conversation though.

This is an important book.  Ernest Hemmingway, another famous American writer, had an opinion about it.

All modern American literature comes from one book by Mark Twain called 'Huckleberry Finn.' (pbs.org)

I always have my students discuss this idea.  It really gives them perspective.

This is a surprisingly complex question and one very seriously addressed by Wayne Booth in The Company We Keep: An Ethics of Fiction. Booth's position is that ethical criticism must reenter literary criticism, and works that are not ethical or that present unethical positions held by characters--which may and do become confused with the author's own contrasting ethical position--need to addressed as such so that instructors and students will be conscious of the harmfully influencing power within unethical and/or bi-ethical (the characters / the author) texts.

Booth cites authors like Flannery O'Connor as (what I call) bi-ethical authors (the character / the author) and texts like Huckleberry Finn and Heart of Darkness as (what I call) bi-ethical narratives. He makes the point that human cognition is metaphor based, thus narrative metaphor can be confusing and even overlooked if instructor and student are not prepared for the narrative's power to harm.

Booth tells an anecdote of his earlier professorship years when an African-American professor declined to teach Huckleberry Finn on the ethical grounds that it harmed his students. Though vigorously opposed to the professor's decision at the time, Booth came to agree and cites this incident as part of the impetus for his call to reclaim ethical criticism. I agree with the professor of Booth's anecdote and with Booth: some novels should not be routinely taught because of the harm they do. Huckleberry Finn, To Kill a Mockingbird and Heart of Darkness are three titles that, under accepted ethical criticism, should not be taught.

I have to echo the thoughts of the above editors. I do not see any problem with teaching Huck Finn in school. What I do think needs to happen, prior to reading the novel, is an examination of the period and use of language during the time. This way, students come to understand the difference between the use of questionable language in the novel.

Huckleberry Finn is a deserved classic, and one which speaks far more to the fundamental rights of human beings, whatever their skin color, than to the superficial and entirely fabricated outrage over the era-appropriate language. Now more than ever parents should know that their children are exposed to every possible racial and ethnic slur through the magic of the Internet; most of them use it in their daily lives. Just look at all those YouTube videos of gaming kids "raging" for various reasons; their language is far worse and more vicious than the casual slang presented in Twain's writing. Because slang is exactly what it is; it's no different than the slang used today. To censor it or try to ban it is to refuse to remember history as it happened, and instead fall prey to phony nostalgia for a time that never existed. In fact, with the extraordinary amount of writing, music, and film that exists using those words in a purely vicious manner, Huckleberry Finn is almost innocent in its use.

I do not really have any trouble with the idea of teaching this book in American high schools.  Anyone who does not realize that people used to use the word "ni--er" all the time back in the old days is hiding their head in the sand.  Sure, it's offensive today, but surely no more offensive than reading in history class about African Americans being held in slavery.

I am not a literature teacher and therefore I cannot really talk about what makes the book important.  But practically everyone says it is an important book.  Assuming that it is, there is no reason to allow the repeated use of one offensive word to prevent the book from being read in schools.