T.J. says that Cassie was Uncle Tomming Lillian Jean because she acted overly eager to be nice to her.

Cassie’s family has taught her that she needs to be nice to Lillian Jean and treat her with respect because she is white and Cassie is black.  The phrase refers to the novel Uncle Tom’s Cabin, and how the black characters treated the white ones.

Cassie offers to carry Lillian Jean’s books, and tells her that she accepts her lesser status.

“… But my papa told me it don’t do no good sitting around being mad. Then I seen how things was. I mean, I should’ve seen it all along. After all, I’m who I am and you’re who you are.” (ch 8)

Little Man says that he is going to tell on Cassie and she will get in trouble, but Stacey tells him that it is between Cassie and Lillian Jean and he should stay out of it.  He tells T.J. not to say anything either, which is when T.J. says he does not care about her Uncle Tomming.  Cassie is annoyed by the slur, but she keeps silent.  She has her own plans with Lillian Jean.  She is not selling out.  She is plotting revenge.