You should not teach Heart of Darkness without at least considering Chinua Achebe's well-known lecture "An Image of Africa: Racism in Conrad's Heart of Darkness." Apparently, few teachers are aware of this argument.
Reevaluation of our core materials in light of contemporary political and social movements is common and can catch teachers off guard.
This lecture is an effective argument, even if used as an example of literary analysis. Digesting it can shape study of the novel and forestall complaints and problems, if only for teachers. I work it with my students and it is very valuable. It leads to thoughtful and effective discussion of the novel in a modern context.
The lecture is freely available online, but that version has many machine-reading errors, is in a cramped format, and lacks context useful to high school students.
This version has been proofed and includes a set of discussion questions and some glosses and context. Includes both a pdf and a .doc version with questions embedded in the text in boxes for printing, and versions without questions.
Beware--the lecture includes the n-word, quoted from Conrad and used by Achebe (though ironically). I've included versions that fuzz the word and those that are as published.
Also included is a word doc of the novel, which is in the public domain.