I was abused at school-through primary school and highschool. To this day I haven't moved on from it, partly due to the added abuse at home. Please help her however you can because there are hierarchies that she will fall into as she gets older. The things that are happening now could seal her fate in the way she gets treated as she gets older. If it keeps happening over a period of time it could get worse and could end up involving more people. That is what happened to me. I had people I didn't even KNOW come up to me and say things that others were saying, it was humiliating. When I was about 6 a boy in art class came up to me and grabbed me around the throat and held a stanely knife in my face (although he was always adamant even 5 years later that it was tin foil, as I think yall be calling it in the US).
I always missed a lot of days at school too. I still have a diary from highschool and one year I had over 20 sick days because I was so sick, stressed and exhausted. I hated school.
Of course there WILL be a reason this girl is acting this way, whether it's because she is a little brat that is allowed by her parents to get away with things, or a medical reason.
Perhaps make an appointment with the principal and voice your concerns, this is not fair for her at all. It's affecting her confidence, her future and her education.