I hope you get the parents to take action and seek counseling for your niece.
Hopefully the school can sort out if there's bullying involved against your niece--I think there's been a lot in the last 10 years that proves physical and verbal harassment against children in school has to be nipped in the bud. In my daughter's school last year, it got so bad for one little girl, and the school was not responsive, so the mother took legal action and it was resolved.
Meta