How to tell my mom I don't want to carpool with...?

My mom and I don't exactly get along, for one. I've been abused by her, but it's getting better and I'm glad. I left my old school because I was being bullied so badly and I came to a new one this year, where my "best" friend was. Now, she basically is bullying me, and we normally carpool with her to catechism (our parents are making us do it for the rest of our school years). She's just been so mean and hurtful; how can I talk to my mom about not driving this girl and her sister to class anymore??? Thank you and God bless!