In Roll of Thunder, Hear My Cry: What is Mama teaching when the school board members come to visit?

Mama was teaching a history lesson about slavery. She was talking about its cruelty and the economic benefits that the ruling class got from the free labor of others. When Mr. Granger opened a student's book, with the paper pasted on the front cover, he accused Mama of teaching things that weren't in the book, which was approved by the School Board. He fired her.