Few of them will understand it, even if they have an expert teacher, and the chances of that are zilch.
Furthermore, Mark Twain's "dialect" is abominable, and children should not be exposed to this "Negro dialect", use of "the N-word", etc., presented as great literature (which the book is, dispite the "dialect") and without adequate guidance, which they are highly unlikely to receive.
Unless this book is explained adequately, it is likely to reinforce racial stereotyping. The subtleties of the bitter satire can easily escape the casual reader, and how many seventh graders--or high school students--are anything but casual readers.
I think it should be studied in college, maybe high school if the students and teachers are serious, but not to students any younger than that.
I know it's a crazy thought, but perhaps Twain wrote the book to be read, not studied
But perhaps to make everybody happy, it should be law that possession or reading of Adventures of Huckleberry Finn should be prohibited to anyone below the age of 18.
heh.. that should do it, they'll all read it then