The sixth graders are “David” and the eighth graders are “Goliath” because the sixth graders are the underdogs, since they are two years younger.
David versus Goliath is a common way of describing a big guy versus a little guy. The David versus Goliath reference is an allusion to the story of “David and Goliath” from the Bible. It often refers to a perception of unequal strength where the one will less perceived strength wins over the won who supposedly has more. In this case, the teams in the Academic Teams competition are unequal in strength.
Mrs. Olinski’s sixth graders were David versus eighth grade Goliath, and the kids with the slingshots knew how to use them. (Ch. 6)
In the story, little David defeats the giant, Goliath, by shooting a slingshot at him. This shows that a little guy can defeat the big guy by being clever. This is why the sixth graders are described as having their slingshots ready.
Since the sixth graders are younger than the eighth graders, they would seem to be easy to beat. However, Mrs. Olinski has carefully chosen the students on her team for being able to not only answer questions correctly, but also work together. She knows this is their advantage, the way David beats Goliath.
One of the beautiful things about the book is that it demonstrates that by working together, anyone can accomplish anything. While the honor and status the academic competition gets in this story may not be realistic, it shows that being smart is important, but not as important as teamwork.