I don't think they teach huck finn in American schools any more becuse it is rather racist.
That was one of my favourite books in high school and I consider it far from racist. Showing how slaves were treated is not racist and having Jim as one of the most human characters in the book certainly is not racist. One of the central themes of the book was the evolution of Huck's point of view from the unthinking acceptance of that period's attitudes toward blacks, to the belief that Jim was better than most of the white folks Huck knew. The only thing I can think of for it being thought racist was the use of the pejoritive that apparently cannot be used even to make a point about the inhumanity of using it. Silly.