Have to have a grade

     "Anyway," he said, "I had them write this term paper. In class." He looked at Jerry North. "It's the end of the winter term, you know," he said. "They have to have grades. I have to find out which have been listening, or even thinking. So they write these papers."

(from Murder Is Served, by Richard and Frances Lockridge)

Which raises the question, Why do we have to have grades?