I agree with everything that has been said. Absolutely take on the school and make them see that this is a problem, and you will not tolerate it. Something I wanted to add though...if you don't get a chance to spend very much time with her alone(which I know has to be hard having 3 that need you constantly), try to edge out a little time for just you and her. Take her out to lunch on a weekend, just you and her, or something similar.
It's possible that being away from the house, and away from the other children, she may open up a little better. I would think that at that age, a child wouldn't want his or her siblings to know what they are going through, for fear of embarrassment and being picked on by them. Not to say that they would, just that it may have crossed her mind and caused her to clam up some at home around them.
Just my 2cents. I truly hope you can get the schools to do something about it. I was the victim of horrible teasing and some abuse by other children when I was in school, and it can be devastating to self-esteem. I still struggle with my own self-esteem and mainly because of trying to overcome the taunts that were pounded in my head by my abusers. I wish you all the luck in the world!