V.W.
When I was younger, I was never afraid to fight back, no matter how big the other kid was. I did however have a bullying situation in elementary school with some WAY older kids....I just got my sister who was in high school to come meet me after school one day. I never had a problem with those girls again!!! I was always taught to not only defend myself, but to win. My older sister taught me how to fight and take down bigger kids as well. My mother taught me to only fight if someone touches me first. Defending myself was not a problem, but starting a fight would get me a whooping at home.
I would go to the school and request a parent meeting with the other girl parents. I would be EXTREMELY firm, but respectful, and let them know that it stops NOW. If not, I would inform them that I would file a police report.
If it's just one girl bullying, then I would advise my daughter to defend herself...BUT if it's more than one girl, I would definitely step in instead of letting her possibly get jumped.
Hope my advice helped somewhat.
V.