Why couldn't Jem play football?

Scout describes Jem as being "football crazy" and spending the majority of his free time reading football articles and playing touch football with his father outside. At the beginning of chapter 26, Scout mentions that Jem is in the seventh grade and went out for the high school football team, but he did not make the roster because he was too slender and too young. Although Jem is too skinny and young to play for the high school football team as a seventh-grader, he enthusiastically fulfills his role as the team's waterboy.
After Scout takes note of her third-grade teacher's hypocritical comments regarding prejudice in America, she attempts to discuss the situation with her brother later that night. When Scout enters Jem's room, she notices at least twelve banana peels on his floor and an empty milk bottle by his bed. When Scout asks her brother what he is stuffing himself for, Jem says that his coach told him that he needed to gain at least twenty-five pounds in order to make the team in two years. For the time being, Jem enjoys being around the game and is glad to help carry water for the team.

In To Kill a Mockingbird, there is evidence of Jem's love for football throughout the novel. The reader learns right away in chapter 1 that Jem has an interest in football. Scout references a broken arm which causes Jem to worry about his chances of playing. Chapter 10 informs the reader that "Jem was football crazy." Jem asks to play tackle football with Atticus. Atticus, however, responds by saying, "I'm too old for that, son." In chapter 23, we see that Atticus uses a new football magazine to get Jem to express what may be bothering him. It is also in this chapter that Jem shows his new chest hair to Scout. He then tells her that he is “Goin‘ out for football next year."
In chapter 26, the reader learns that Jem is in 7th grade and part of high school. Scout informs the reader that "he went out for football but was too slender and too young yet to do anything but carry the team water buckets." Although he is not yet part of the team, Jem is happy to be part of football.