Why does Harlan granger show up at mama's school?

Kaleb Wallace, another man, and Harlan Granger enter Mama's classroom. They say that they are representing the School Board and watch as Mama continues the history lesson she that she had been teaching about slavery. Mama talks about its cruelty and the economic benefits that the ruling class got from the free labor of others. Mr. Granger opens a student's book, with the paper pasted on the front cover, and accuses Mama of teaching things that aren't in the book, which was approved by the School Board. He fires her.