It is kind of hard to say too much about this, without knowing all the circumstances that lead up to this happening. The child could have been a bully, but then again, so could the teacher.
The child could be mentally retarded. The teacher could be abusive or at the very least scary. Maybe the child was being bullied by some other kids and took it out on the teacher. There are any number of reasons this might be happening that have nothing to do with bad parenting. But, not acting on it until it's happened 4 times and then having a 9 year old arrested is rather irresponsible on the part of the school. I wonder why they didn't try moving the child to another class or sending him to a therapist to find out why he was becoming so violent.