^This! Very well explained.I can see where they were trying to make the Joke that Human are primates and so part of the same family as other great apes, and yeah, they might have done so with many other students all whom were white and it was never an issue. However that is because white people haven't historically been referred to as monkeys in a derogatory way. By putting four black students in the image, rather then a mixture of black and white, or only white, then the image plays into the stereotype and racial slur. It's like the difference between a picture of a white man standing in front of a tree with a noose hanging from it with the words "hang 'em high", and a picture of a black man standing in front of a tree with a noose hanging from it with the same words, or a picture of a man cooking with the words, his place is in the kitchen, compared to a picture of a woman with the same words.
It's a case of the connotations of the image and words, and these are different for white students and black students, and the teachers should have known this, and avoided them at all costs,. They didn't and now it's going to cost the school district and it's insurer a lot of money as punishment.
Something okay when applied to one group can easily be racist and deeply offensive one applied to another. Absolutely!