My suggestion is for her parents to do several things right away -- I am presuming that they have met with the school counselor or the principal in charge of discipline and that perhaps some kind of mediation has been attempted. The next step is to set up a meeting with the principal and come prepared with a list of all the times, places, and people that have been involved in harassing their daughter. Calmly, but firmly they should tell the principal that the harassment must stop and ask what steps the school will take to make sure that it does. After discussion of what will be done -- and this might be many things -- they should ask "when, where, and how." Lastly, ask for a meeting in a couple of weeks to assess how the plan is progressing.
As for harassment taking place off school grounds -- especially any physical acts such shoving, pushing, and spitting -- go to the local police or the school resource officer and file an assault report on the students involved. There needs to be a record of the harassment and the victim's steps to resolve the issue. Don't worry about trying to prove it or not. That is the task of the police. It usually only takes one visit from an officer to one child to straighten out such issues.
Good luck to your niece and her parents. Please remind them that every student has a right to a safe and non-threatening education.