Why has Walter Cunningham, Jr. stayed in the first grade for three years in Harper Lee's To Kill a Mockingbird?

In Chapter 3, when little Walter Cunningham comes to have dinner at the Finch house with Scout and Jem, we find out Walter has been in the first grade for three years because his dad keeps pulling him out of school every year to help with work in the field. Because he only attends school for a little while each year, then, Walter never learns enough to pass the first grade and has to keep repeating it. It seems Walter's finally able to stay a bit longer now, though, because his younger sibling is old enough to do the work Walter was previously pulled out of school to do. 

Here's how Walter explains it to the Finch family:

“Reason I can’t pass the first grade, Mr. Finch, is I’ve had to stay out ever‘ spring an’ help Papa with the choppin‘, but there’s another’n at the house now that’s field size.”

So it's certainly not that Walter is necessarily stupid or lazy, it's just that he didn't have the chance to attend enough school days to pass first grade.

Walter's situation reveals how deeply he and his family are in poverty; they put their immediate concerns and needs (like farm work) over the children's education.