I'm glad you daughter trusts you enough to come home and tell you about this situation. Many young girls don't tell and suffer in silence.
You need to contact the administration at the school immediately as well as her teachers. Most schools today have an anti-bullying policy and know what steps need to be taken next. Don't be afraid to stay on top of the situation. Ask lots of questions like:
What steps will you take to make sure my daughter is safe?
What will happen to the mean girls?
Will the boys be told the consequences if they do what the girls tell them to do?
Will their parents be contact?
Do I need to fill out any forms reporting this situation?
Can I get a copy of the plan?
Where can I find a copy of the school's bullying policy?
If your daughter is attacked be sure the authorities are contacted and changes are filed. Whether you go through with any legal action is up to you, but make sure you have a paper trail. (This goes for the school decisions, too.) You don't want it to come down to a he said, she said situation with nothing to back up your story.
Unfortunately, bullying goes beyond the school hours. Cyberbullying often accompanies face-to-face bullying. This can come in the form of phone calls, texting, emails, online slam books or pages, instant messaging, chat rooms, etc. Don't be afraid to check your daughter's Facebook page and her friends' pages and text messages she has received as well as sent.
Ask your daughter what she thinks is motivating these girls to act this way. Did she innocently comment of something they said or did? Keep the lines of communication open. Reassure her that you are there for her and will do whatever you can to protect her.