Why is Billy afraid that he won't be smart enough for second grade in the novel, The Year of Billy Miller?

Billy is worried about beginning second grade due to the accident he had a few weeks before at the Jolly Green Giant statue, where he fell from a lookout platform while trying to catch his baseball cap, blown away by the wind. Billy gets knocked out, wakes up in the hospital, and the doctor confirms that Billy is alright, though he will have a lump on his head for quite a while. It leads Billy to think he may not be smart enough for second grade.