LACY question

What is "acting white"? As an educator, I hear my middle and high school students use it daily. From what I am able to gather, they use it to say that a black student is mirroring the way the white students act and speak. Unfortunately, the black students that are using it, are not using the phrase as a compliment. Their intentions are to hurt the other black students. It is also unfortunate that the black students that are accused of "acting white" often find themselves caught between two worlds. But my question is why? How exactly do white people act? And how should black people act? Is their a manual or something? 🤔