Why didnt Walter Cunningham pass the first grade?

There is no evidence in Harper Lee's "To Kill a Mockingbird" that Walter Cunningham did not pass the first grade. The text only mentions that he is in Scout's class and that he is "a year behind" due to being held back.

The novel focuses on Scout's perspective, and the story doesn't specifically address Walter's academic progress. It's possible he was simply a slow learner, or there might have been other reasons for being held back, but the story doesn't elaborate on this.