How old is your daughter? My son went through a stage like this during 4th and 5th grade. We had some pretty bad bullies at this school, but they were so sneaky, the teacher never saw what was going on. I taught my son to stand up for himself, don't hit or call names, but use humor. . .but never back down. Now he stands up to the scariest of bullies, and many of them now respect him. I do worry about some of the things he says to them. (his humor gets a little carried away.)
Good luck and God bless.
Sam, I think it's almost impossible for a child who is meek by nature to stand up to her peers and verbally go against them.
What you CAN teach her, is to quietly go her own way when things are not turning out right. For example, if all her friends are sneaking into an R rated movie, she can pretend to be sick and call you to pick her up.
Specifically, what do you want her to stand up against?