Bullying on school grounds

My child is been bullying at school by a group of children, this started in Oct. Sometime in Nov. a child grabbed her by the hair and dragged her on the floor. The school sent her home with another child, they didn't call or notified me. Spoke with the school they told me it was an oversight and they should have called but didn't. I had to corner the Principal so we could have a discussion about the fight. He finally suspended the child about a week after the incident. I sent a certified letter to the school, March 15, about a recent incident and the Principal acknowledged that he rec'd the letter late Tues. He never called to schedule a meeting. I was giving the school sometime to respond because of spring break. The school had the letter for five business day, and they didn't even called to address my concern. I demanded a meeting and he scheduled me for Monday April 11th. Other then document and call the school, what else can I do? Why do schools take so long address bullying?