These boys start out as Julian's friends. Julian is a popular kid and they're his posse, his unquestioning minions. And while they don't do anything overtly mean to Auggie, they sure don't look for opportunities to get to know him—and they do harass Jack, stealing his gym shorts, crumpling up his worksheet in class, dumping pencil shavings into his backpack, and such. Why Jack instead of Auggie? Because they know they can get away with it. Classy.
But the tyranny of Julian's war against Jack (and indirectly, Auggie) wears thin over time, and most of the boys lose interest. Amos is the first of that group of boys to actively stop participating in the bullying.
When Julian decides to skip the too-dorky nature retreat, the boys are all free agents suddenly. So when August and Jack run into to trouble in the form of some big, mean seventh graders in the woods, Amos and Henry and Miles come back to check on them and wind up saving their classmates from a really ugly scene.
These boys finally do the right thing, probably because there wasn't someone there telling them not to. The proverbial light bulb finally goes on for them: the right thing is to look out for people, to protect kids you know from kids who would hurt them—no matter how anyone looks.
They're minor characters, but they undergo some major change and—in doing so—remind readers that we can always make different choices going forward.