school harrasment

My daughter is being both verbally and physically bullied at school to the point where she has cried every day from the time she got on the bus till she gets home. Then bulling has been following her home at the end of the day on the school bus. She makes up excuses not to go to school and begs and pleads with us not to make her go. There are several girls involved. One girl has been rubbing her back side against my daughter and pushing her into walls using her butt. One of the other girls has been pushing her around and verbally assaulting her. This girls mother and older brother have been calling both my daughters and on one occasion the brother actually threatened violence against my ten year old. The same mother and brother actually came to my home one night around 10pm and threatened me and my daughter on my own property. I had to get the sheriffs department involved so she would not come back. My daughter was called in by the school Principal and was berated as being the aggressor after one of the girls made a complaint that my daughter pushed her in the hall way. This is not the first time my daughter has been bullied at this school. And we actually talked to the principal about it once before and it has got worse. And after talking with the principal today she was in no way assuring me that she was going to look into it.