Are girls in middle school and HS are really mean to each other?

I know it's silly but I just saw "Mean Girls" and read a book about "Queen Bees"...and it is so scary to know that girls can be so ruthless. Is this really true out there? My daughter is only five but the cliques and meaness between girls start I heard in middle school. What have you done or said to your daughter to prevent them in falling into that kind of pressure. I know that some parents also encourage that kind of behavior too...What kind of advice can I give my daughter so she becomes a confident nonfollower individual.