What did Stacey explain to Cassie about Big Ma?

Stacey didn't really get to explain..... Cassie was too bust complaining. What he tried to explain was that Big Ma reacted to Mr. Simms actions in the way she believed best suited the situation. Had she reacted angrily and told Mr. Simms off..... refused to have Cassie apologize to Lillian Jean, there might have been trouble for the family. Big Ma knows what kind of trouble could have been stirred, and she chose to "protect" her family above all else.