Through the course of the book they must learn to see past their differences and labels, ( The slacker, the slut, the overachiever, etc.) which is a central theme in the book, and figure out how to become something more greater than what society has them labeled as.
I feel this book is a meaningful one for just that reason. Everyone in life has been assigned some sort of label and these labels hold us back. The stereotypes behind these labels make us feel like we have to act or present ourselves in a certain way to be accepted in the world and acting outside of these unspoken rules is forbidden.
At the beginning of the book you don't understand how these teens could ever find something in common, because you know they do, they're the main characters. But by the end you realize they have almost nothing in common, the labels are still there, but they manage to overlook what separates them and come together and find solace in their friendship in the end.