I can still remember how it felt. I was held back in the 6th grade (unlike Joshua, in my case it was my own fault), and though Joshua didn't realize it, he was actually very fortunate to have been held back earlier rather than later.
Susan Shreve must have been held back herself as a child, or had at least one of her own children held back, or dealt with many students who have been held back. She writes the story in such a way as to meet the reader right where they are. She seems to identify with someone who has had to repeat (or is in the process of repeating) a grade for whatever reason.
Although it is a very short story (initially written for my generation -- see the "Pac-Man" refernces), it still brings home some very important messages for young kids struggling in school, whether it be academically or socially. Also, in the tradition of Katherine Paterson's "Bridge To Terebithia" and Louis Sachar's "There's a Boy In the Girl's Bathroom", this story also does a wonderful job of offering us the character of a teacher who is able to reach the student on a deep and meaningful level...the kind of teacher we never forget about no matter how old we get.