"Why can't we discuss?"

Returning from a tough social sciences paper, my son was asked by his grandfather- "How was the paper?"

7th grader: "Not very good. Geography was ok, but not history and civics."

GF: "Did you discuss the answers with your friends after the exam?"

7th grader: "No. What I can't understand is this - though the paper includes questions that begin with 'Discuss the following', the teacher does not let us discuss...."