What makes Marguerite so angry with Mrs. Cullinan? How does she protest?

Mrs. Cullinan treats Maya rudely and refuses to call her by her proper name, she calls her Mary instead of Marguerite,  Maya begins to strongly dislike Mrs. Cullinan.

Maya decides that she has to quit, since she cannot bear to be renamed by Mrs. Cullinan, simply because a shorter name is more convenient. Maya can't just quit, though, because her Momma would not let her; so, she decides to drop Mrs. Cullinan's favorite dishes, and get fired because of it. Mrs. Cullinan yells at her, and finally calls Maya by something close to her proper name, Margaret. Maya leaves, happy that Mrs. Cullinan finally admitted that her name wasn't Mary.