Rosalind, it might help us all if you can give a little more detail on the situation. Some questions not just for you to answer (you don't have to!) but also for you to think about as you consider the situation:
Why does she need to be at this this particular school (one person who posted assumed that you had money and could "let your money do the talking" but your old posts indicate that is not the case at all); are there reasons she cannot live with you and go to a local school? It's easy for us here to say "Bring her home" as some have already, but you may have some very compelling reasons she needs a boarding school environment.
Why does she say she specifically likes this school overall, before these issues got going? Does she do well academically there; do you feel comfortable with the adults who run it (other than in this case); does she like what she's learning? Or do you feel she might possibly be saying she likes it there mostly because she's familiar with it and she fears the unknown if you were to send her elsewhere? You need to talk to her more and find out if she is truly happy other than with the bullying, or if she is mostly fearing being pulled out of the school because she doesn't know where else she'd go.
Does the school do anything other than talking about bullying? Is there any actual punishment for the bullies themselves? Privileges taken away, and/or unpleasant chores assigned, for instance? Did the other child involved in the shoving match also get suspended for it, or only your child? (if the other child didn't get the same discipline -- big red flag!) The case manager "talking to the girls and calling the parents" sounds weak to me; the school should have a firm system of removing privileges when students behave certain ways, even verbally. If all that's done is talk, nothing will affect these kids.
Is the case manager's or house parent's personality part of this? Perhaps she's a softie type when previous case managers or house parents were tougher and kids knew they could not get away with things?
Can you get to the school yourself, in person, or is it too far? Can you turn up unannounced? I would not put a child at any place where you were required to let them know you were coming -- it's the same principle as with a preschool: Parents should be able to drop in any time. If you have to let them know you're coming, they have time to "clean up" the situation and get their stories coordinated.
Bullying over a kid's size has happened since the start of time, but it's much worse in this case because your child must sleep and eat with her bullies. You can give it some more time -- after talking to the school in person about whether they are DOING anything with the bullies or just talking to them. You also can role-play with your child the scenarios where she is being bullied and give her constructive things to say, and teach her how to walk away (again -- harder in this case since they live together every day). I would definitely ask the school counselor to meet with your child and give her specific techniques for how to react to these kids so she is not getting into trouble when they anger and upset her.
I feel so sorry for her, and for you being away from her during this time. Push the school hard on actually disciplining these other kids, and work with the counselor closely and immediately to teach your child to defend herself now and in the future. And meanwhile, investigate other options such as other schools or bringing her home IF you and she and the other adults at the school cannot deal with this. But I would not pull her out instantly. First work on giving her tools for defending and deflecting, and ensure she feels very good about who she is and her own talents and personality. If the counselor, case manager, house parent, and other adults are not cooperative with you -- I would let them know very clearly they are just condoning bullying and seem to fear their own students. I hope it does not come to that, and they are concerned and cooperative!