Are teachers allowed to join in mobbing a pupil?

Tweet

I'm asking this because my girlfriend is always mobbed in school and the teacher is even mobbing her then too instead of stopping the other pupils. She can't even go to an other school because she is in her apprenticeship and it's her trade school and doesn't have a choice. So I wanted to know if we could sue the teacher because my girlfriend can't take this much longer before she breaks down and I can't even help her and that makes me slowly sick too.