Not sure if this is the correct forum, but it is somewhat education related.
And fair warning, if you laugh at this that means you're totally racist.
I'm not enraged. I am a racist and so are you. I am not a bigot, I'm not sure about you. This overreactive mother seems very much like a bigot.
So you have a teacher, in a moment of insensitivity, is testing the pupils' understanding of word relationships. Was the teaching even thinking of human slavery?
Or
In this teacher's subconscious a variance of the following:
When I was six, my grandfather started teaching me how to read and notate music. (you can look this up for detailed explanations) At my age seven, we had reached a point of instruction where he was teaching me the use of clefs. Clef is a French word, meaning a musical key which is used for indicating pitch. When discussing the use of double clefs, as in dual treble clefs, one was the masterpiece, the other the slavepiece, with triple trebleclefs, one was the masterpiece, the other two the slavepieces.
Over the years I have learned similar key theories for everything from notations for color schemes by art and moving image teachers notating light effects upon color as elements for creating mood. Masterpieces and slavepieces, to engineers using notations for ordering the physics of design when notating transference of power transmission. The same for many other fields.
Like the ignorant substitution of racism for bigotry by people who understand neither or the use of language for precise depiction of thought and ideas, I give you no fair warning for convincing me you are unthinking, just like this overreactive mother.
Perhaps this will help you? "Black is beautiful" is a positive racist statement, it is not a bigoted statement. Racism can be celebrated like all ethnic qualities, bigotry, in any direction, never accepted.
When suggesting to another overreactive parent that she use the novel Tarzan to encourage her recalcitrant child to read because Tarzan teaches himself to read, her son having recently seen an animated Tarzan movie and being enamored with it, she responded with "I can't do that. That book is filled with racism."
I laughed like I am laughing at you and this parent's overreaction now. I responded to her, "Terrific, as he reads the novel, you will have talking points to discuss these elements, why people developed these attitudes, why many people still have these attitudes, and why they are unacceptable. You know, the truth as you see it, and listen to the truth as he sees it." When you hide children from real life, you lose any trust they may have given you.
So take your hubris and stuff it where the moon don't shine, along with your faux outrage, and hers. Seize the moment, seize the opportunity.