I agree with that, with the caveat that if you're teaching it in school you should be also teaching the context- which doesn't mean just saying that "oh, people talked like that in the old days".
There was one version that tried to simply omit "nigger" whenever it occured, resulting in a dialogue going:
"Good gracious! anybody hurt?" she [Aunt Sally] asks.
"No'm. ..........."
"Well, it's lucky, because sometimes people do get hurt."
where the missing line is "Killed a nigger," which serves for Twain to point out that a nice old lady such as Aunt Sally doesn't even consider a black person as human. Even replacing it by 'slave', as some versions do, weakens the point, which is it was the colour, not the condition, that mattered.
Likewise, in the scene where Huck decides to write a letter to Miss Watson to tell her where Jim is, because his conscience is bothering him about " stealing a poor old woman’s nigger that hadn’t ever done me no harm" [the old woman, that is], followed by
“There was the Sunday-school, you could a gone to it; and if you’d a done it they’d a learnt you there that people that acts as I’d been acting about that nigger goes to everlasting fire, ” that is, he was going to Hell for trying to help Jim escape. Here Twain is attacking all those good Christians who supported the whole system of racism and slavery.
The question is, how often and how deeply are those aspects taught? Or are they bleeped over to go on with this wonderful adventure story?
