@JufusMum I'm sorry to read that you DD has been bullied. Is she going to a new school for sixth form? I hope so because bullying is awful at any time but especially bad in these formative years.
It sounds as though the maths teacher has given up trying tbh. I can see that you don't need to be able to do everything in order to get a good grade and its better that the class learns some skills well than all skills badly, but I don't think its ok to be taking this approach without consulting with parents.
Does your DD struggle with maths?
Mr Hegarty Maths (youtube videos) are supposed to be quite good at explaining stuff. TBH though, maths is just a series of skills that can be honed through practice, so once your DD has learned the theory its just about doing lots and lots of examples.
I hesitate to suggest it, but maybe a tutor would help if there is a self-confidence issue?