what does scout learn about calpurnia?

In Chapter 12, Scout learns that Zeebo was Calpurnia's son and taught her how to read and write. Because of this, Calpurnia speaks differently at church among the other blacks than she does when she's with the Finch family explaining to Scout that to speak in her perfect English to friends (the way she speaks at the Finch household) would be considered "uppity".