Ethics! Psychological experiments don't just get to abandon it. This is unethical, and unhelpful. Racism doesn't work like this. As the kids said at one point, "I feel like my best friends were taken away." With REAL racist societies, the kids are separated from day one, and they don't build up friendships, ignorant of any real difference, only to be told by the teacher one day that they're different and inferior/superior. So this experiment doesn't help us understand racism really at all. Plus, it was clearly harmful to the children. She even mocked a boy in front of the class because his father hit him. Not funny, not good science, just bad ethics.
Racism, classism, religious fundamentalism. It is so frightening just how easy social manipulation is. No wonder American politicians often combine religion and politics. Do you think they do this because they're nice?