The former encourages children to learn about the world around them (and the world the way it used to be), which is not always a sunshiney beautiful place.

The latter drains all the joy out of a story and guarantees that the child will never pick that book up again.

You can (and should) absolutely talk in class about racism and sexism and classism using literature as examples, but if you make it so that that is the only situation under which a child can have access to that literature, you've completely destroyed everything good about the book and reduced it to its negative aspects.