Why does Billy worry that he will not be smart enough for second grade in the novel, The Year of Billy Miller?

When Billy and his family visit the Jolly Green Giant statue in Blue Earth, Minnesota on their way home from Mount Rushmore, Billy falls from the platform. As a result, he hits his head, and ends up with a large lump which he proudly shows to Ned. Yet, this also causes Billy to worry that he will not be smart enough for second grade.