.sfw121006 this sounds really awful for your dd. and also for you.
I have not been through this but have read and heard a lot about bullying.
There is an anti-bullying section of Mumsnet for advice.
There are also anti bullying charities that you can contact for advice.
How old is dd?
How long has this been going on?
Please take your dd through the steps to get a referral for anxiety via GP. Whatever has started it she needs help. Could this be used as a reason to keep her off school for her to have some build herself up time or would this make it worse?
Bullying can diminish a person so do all you can to build her up. Let her have her say about what you should do and say. You may not give her, or be able to give her, full autonomy but you can give her some autonomy. Work out what choices you can allow her before speaking about this, I suggest.
In your shoes, I would make an appointment with teacher or head of year or head (whoever you feel will be most sympathetic/understanding/ get most done.
Ask for this appointment as a matter of urgency.
Get the full picture from your dd about what is happening. Take your husband/ partner or a trusted friend or relative to the meeting, take a pad and pen and make notes. Whether your dd comes to meeting or part of meeting is really dependent on her age, IMHO.
Ask school for their take on situation, what they are doing, what the plan is etc? Then say how you would like things handled and see what they suggest.
Government information and guidelines on anti bullying are available on line.print these off, read them. Print of school anti bullying guidelines and read them too.
Is your dd in physical danger?
If this confines I would be prepared to move school.
Good luck, stay strong. Your dd beds you to be strong and not overwhelmed by this 