Joe Alvarez mentions the specific case that I was trying to recall.  But
this attempt to "doctor" Twain's text is not restricted to print
alterations.  I've had high school teachers tell me that their department
chairs or principals instructed them that they were not to use the "n" word
in discussion of the book in class.  In fact, they were instructed to begin
the unit on HF with an explanation to the students that they would not be
using the word, and when they [student or teacher] read passages from the
text, they were to substitute "slave" for the offending word.  When they
pointed out that Pap rails against a "free nigger" from Ohio and that
substituting "slave" for the "n" word wouldn't make any sense; they were
told not to worry about it and simply avoid upsetting students, and their
parents.

Reminds one of Twain's aphorism:

In the first place God made idiots.  This was for practice.  Then he made
school boards.
                        FE, Chapter LXI

Larry Howe