Been there, done that. I never got further than outside the bus near the stairs, to talk to the bus drivers(they have to learn towards me to speak and hear). I have also had a child last year(who was taller than I and weighed more than my son) say that my son was starting fights, that I was scaring him by leaning my head in the bus to hear the bus driver. He also said the same thing when I talked to her through the window.
The parents refused to talk to the driver, and said that their son was such a good boy, and these "bullies" needed to be kicked off the bus. If she had kicked everyone off for getting into fights, he would have been the only one left by the end of the year.
After hearing this child was involved in "fights" with six other children(told to me by their parents), I went to the principal, and the superintendent. I refused to let my son and these other children be punished because one child who was involved in all the fights was a bully. They called all of the other parents, got confirmation, and then assigned him the seat opposite the driver in the front(because he's even hit the driver). When the parents complained, they were told that it was either that, or the parents had to transport him to school every day.
He has since gone on to sixth grade, started trouble on new bus, and was given..yup, you guessed it! The same seat arrangement from last year.
Talk to the driver from the outside window only from now on, get her direct extension at transportation(when you call transportation), and also call the principal every time this happens from now on.
A bully will always hide the truth, and most kids are too afraid to speak up. If enough reports from every parent that's had a child bullied by her are logged, they can arrange a special seating arrangement, parental transport, or other disciplinary measures.
That said, it wasn't clear with your information who was the aggressor. If the other girl had "marks" on her face, and not your daughter, what happened? If it turns out your daughter started it, will you punish her?
Just my 2cp.