Throughout the short story, Laurel (also known as Snot) and her fellow troop members are filled with indignation that the white girls who show up at their campsite have addressed one of them as "nigger." Plans for revenge are plotted and executed, after which they discover that the white girls from Troop 909 have varying levels of learning disabilities and special needs.
I would argue that the message about race—and about life—that Laurel learns is that racism is something endemic to the world. In thinking about the events that took place at camp, she realizes that racism is often fueled by misunderstanding.
She recalls the time her father had a Mennonite family come and paint their patio. The Mennonites will do whatever is asked of them, with no expectation of payment or thanks. When Laurel thinks about this again, she realizes that her father took pleasure in having a white family do work for him, and it makes her uncomfortable:
I now understood what he meant, and why he did it, though I didn't like it. When you've been made to feel bad for so long, you jump at the chance to do it to others.
In a nutshell, she learns that racism breeds further racism. In watching the Mennonites paint his porch, Laurel's father reclaimed some of the pride that had been taken from him by white people over the years. It is a depressing realization for Laurel, who now understands that there is "something mean in the world that [she] could not stop."