You cannot make sure that this student gets expelled (permanently excluded). You can hope that the governing body upholds the head teacher's decision to permanently exclude her. Her record of previous assualts etc will go against her and hopefully these will be taken into account by the governors. A letter to the governing body may help. Contact the police and ask them what you should do with the crime number you have been given - clearly they must think the incident is serious. However, it depends how old she is as well.