Go and see the head. It always amazes me in discussions about a teacher's performance how much agonising there is about whether we can possibly excuse the teacher's behaviour.
This is unacceptable. If the teacher hasn't worked out that it's not her job to put aside personal likes / dislikes as part of the quid pro quo which involves her being paid for her time, someone senior needs to point this out.
I'm not at all surprised about the maths thing. When I changed school when I was 8, I was about to finish book 4 of whatever the book was in Maths, and about to start book 5. Two things happened.
In my old school, I wasn't allowed to move on to book 5 (for about 3 weeks) while the teacher's favourite pupil caught up with me (or he'd get upset). And then when he was ready I had to wait a further week to give him a head start.
In my new school I had to go back to halfway through book 4 as this was the furthest anyone had got in my new class - because if I was put on book 5 it would - you guessed it - upset the other pupils.
I also utterly disagree with the poster who blamed the other mother for reporting back what she'd heard about the "nightmare pupil" comment. In the other mother's place I would have done the same because I would have considered it unacceptable and would have thought the mother had a right to know.