I know from a teachers point of view it is VERY hard to stop bullying, thats why with the best intentions and hard work, it still goes on.
I do beleive that kids need to take some ownership of the situtation... I'm not saying you need to tool up your daughter so she goes in and batters them... but if you can help her deal with the situation herself it will be far more empowering and longer lasting.
Helping to raise kids self confidence and sense of what is right and wrong helps - and I'm not saying you need to directly do this with your daughter - I mean as a school thats what they need to be doing. I have lots of bullying issues with my Y8 form, and one of the ways we deal with it is openly discussing it in the class. I try to develop a sense of shared responsibility. For example, this week some of the other Y8 boys created a Facebook group (a whole new side of bullying that is VERY hard to police) about a boy having a small penis, and an event where they were going to beat him up. A few members of the form had 'accepted' the event, so we discussed how that would make people feel (the boy in question was off as he was so upset), and actually the class ended up having a go (in a controlled, classroom way) at the people that joined, those who did joined apologised to the class, and when the boy returned they literally all welcomed him back with open arms. I'm not saying for one minute I have all the answers, but making kids realise what they are doing is wrong and why is key. Most of the time they just don't think...
Back to your daughter, I think it is important to inform the school, but don't expect a radical, visible solution. Often there are things I just don't know about! Equally for a child, just to tell a teacher really helps. Schools can then build up a picture of who is a bully and who is being picked on and work out ways of dividing to conquor the bullies, helping them to see the error of their ways, or as sometimes happens, being a little more understanding when some one 'let's rip' and punches a bully (again a scenario I had, a boy that I knew was getting some hassle, blew his top and punched a bully in the face... in normal circumstances he would be suspended, but in this case, I took the poor tormented lad and he had a cup of tea and biscuit in the learning support centre - I made it clear that his behaviour was not acceptable, but that I understood why it happened - he didn't get teased again).
Anyway just a few nuggets from me, not an answer I'm afraid, but as said on here, don't lose the plot, be calm and try to work out strategies both with school and your daughter.