Jerry is on the athletic field at the opening of the story because he is trying to make the football team at Trinity School. Physically small and not particularly gifted in the sport, Jerry is nonetheless determined to be the quarterback on the team, and is taking a beating trying to prove himself. Time after time, he is brutally tackled and driven to the ground, and although "he had never felt so lonely in his life, abandoned, defenseless", he keeps getting back up, his body a mass of aches and pains and his mind battered almost to the point of senselessness. The coach questions why a person like Jerry wants so badly to make the team, but he has to admit that the boy "has guts".
By beginning the story with Jerry trying out on the athletic field, the author is telling the reader a couple of very important things about Jerry's character. The very fact that the skinny freshman would put himself through so much reveals that making the team has great significance to him. Later, in Chapter 9, the reader discovers that Jerry feels sorry for his father because his life appears to be so devoid of purpose. Jerry wants more for his own life; he wants to be "a part of something". Being out on the field with his teammates, "bruised and battered or grimy and dirty" as he may be, gives Jerry an all-important sense of belonging, and the sense that his life has meaning.
Jerry eventually makes the team, and in Chapter 12, the coach, with grudging admiration, even suggests that they "might make a quarterback of (him) yet, (even though he is a) skinny son of a bitch". By opening the story with Jerry taking a beating out on the football field, the author is presenting up front the fact that Jerry is tenacious, courageous, and has unusual strength of character, all attributes which will come into play when, in quest of a deeper purpose in his life as a whole, he takes an unpopular stand during the "chocolate war" (Chapter 1).