"It's out there. I don't like it, but it's a fact," Harrison said. "It'd be great if one of them could go, 'Yeah, that is really terrible about our society and I don't want to go down that road.' This is the age when they're all so impressionable, but they are also so aware. I think, by 12, it's already too late. So, I'm not going to write a gosh, golly book just to pretend that this stuff doesn't exist."
It exists - - those toxic pressures to conform, to run with the in crowd -- not just in kids' lives, but in Harrison's too.
She says she gets her material from her world -- the adult world.
What those girls are feeling, their emotions, are no different from what an adult feels, she said.
"I've just taken those feelings and that longing to fit in, that need to be accepted and loved, that need to be seen as smart and attractive, and viable and important, and just dressed them up in smaller, more expensive clothes," Harrison said with a laugh.
But is it a good thing to drill so much of the adult world's obsessions so deeply into children's literature?
Harrison says her books actually help her readers by showing how the apparently glamorous lives of her mean girl characters can be hollow.
"I don't mean to brag -- but I get literally thousands and thousands of letters, thousands and thousands of e-mails from these girls, and I do read them and not one of them has accused me of perpetuating poison into their world and their society," she said. "Every one of them says, 'I suddenly realize that it's not so important to be popular anymore. I used to be like this with our friends, but we've all changed. Truly. I really, really mean it.' They all get it."
During her reading at the bookstore, Harrison demonstrated her point.
"Do you guys get ... that I'm not saying that wearing labels on clothes," she asked the girls. "I'm not saying this is the way we should live our lives right? Do you guys understand the point of this? Would someone like to tell me what it is? I just need to hear it because I'm alone all the time."
Their answer? Yes.
"I would lip kiss you right now if I wasn't so far away," she continued. "And what do you think about girls that obsess over clothing labels. Do you think that's a good thing? Do you guys want to be like that?"
The answer? No.
"Say it again, sisters! Say it again!" Harrison instructed them.
Later, at least some girls -- and their moms -- said "The Clique" books help.
One mother, Robin, said she didn't have any concerns when her daughter Hannah started reading the books.
"I didn't. Only because Hannah had gone through some stuff with some mean girl. And so, I thought it was a really good thing for her to see some other characters and how they dealt with the circumstances that she might have experienced," she said. "We come from an area where there are a lot of cliques, especially in middle school."
Robin said Hannah's experiences with her girlfriends brought them closer as mother and daughter as they looked for answers together. And one place Hannah looked for answers was in Harrison's books.
"I can talk to her about everything," Hannah said of her mom. "I actually talk to her about the books and stuff. It's cool."
So, the girls get it, this new kind of writing for children -- where the old certainties are left far behind.
"The moral of the story is," Harrison said, "accept yourself for you who are."
Accept yourself. For so many girls, that may just be the hardest task of all.