A parent of a child in my class has been classed as 'toxic' by school. She openly moans about the way the school is run (some of it is probably fair, tbh) and bitches to other parents in the playground.
She attacked came to talk to me about what she wanted for her DC. The 'catch-up' lasted for 1 and a half hours 
I politely explained my situation, and said I wasn't going to abide by her wishes (she wanted her DC to receive higher marks than I'd given and be in ability groups I don't believe DC belongs in).
Following school policy, I wrote up notes about the meeting and gave them to the headteacher.
I was then asked/ advised by senior management to bump up DC's scores and move DC's groups in order to keep the mum happy.
I said no.
I was then told that "mum had the potential to make my life very difficult if she chose to, and that I should choose my battles."
I feel I have been professional with the treatment of her DC, and have treated DC exactly the same as the rest of the class.
AIBU to not do what she says, simply because she'll make my life difficult?