I was the kid who was being bullied at school (including on the school bus) whose mum brushed me off and failed to do anything about it whatsoever - and the effects on me have been life long depression, anxiety and low self esteem.
Yes, I blame the bullies and the school (who apparently didn't notice anything), but in some ways I am most bitter about the fact that the one person who I should have been able to rely on to have my back, didn't.
She told me if I ignored it, they'd stop - and was so convincing that I believed that, when they didn't stop, it must be my fault, and didn't dare go and tell her again, for fear of her blaming me. Her attitude towards it made it impossible for me to go back to her and tell her it hadn't stopped. And she never once bothered to ask me if things were improving - the cynic in me thinks she didn't want to know, because if she had known how unhappy I was, she might have had to put asider her interests and pastimes and do something for me instead.
@FrenchFancie - your dd needs you to protect her and support her, and I am sorry, but telling her to ignore it and not rise to the bait is going to make her feel like she's on her own, against the bully.
That said, I do understand why you do not want to talk to the mum, and I think @Molly2016 is right that the best first step is to contact the school - it is school transport, so they have a responsibility for the safety and care of the kids on the bus.