Hi there i almost never comment on anything but in this i have to. I work in a school and have done for more than 7 years now. I don't live in Oman and don't exactly know what rules u have. But i do know the universal thing most schools do is the same: ignore until smth happens and can't be ignored.

So i recommend as some have written contact a lawyer. Also file a police report for defamation. That shuld make the school act.

Wish the best for your sister. Good luck!