"The Ashwater Experiment" by Amy Goldman Koss
Hillary is a teenager whose parents are free spirits. They travel the country in a motor home, stopping in one place or another for a couple of months at a time for Hillary to go to school before moving on. Hillary has never thought much about making friends or putting down roots anywhere, and she is satisfied with living the way she does. Then comes Ashwater, California.
Hillary is set to have nearly an entire school year in Ashwater-the longest she's ever been in one school. Right away she makes two very different friends. Serena is the most popular and sought-after girl in her class. Cass is quiet and bright but has no friends. Hillary finds herself stuck between these two girls, and it seems she might have to choose between them.
Relationships among high school girls are tricky, and most girls will recognize bits of their own high school experiences in this book, although the popular crowd is probably not as catty and nasty as in many high schools.