Re: I'm furious.
I would start with the bus driver. I would ask the bus driver tomorrow mornign what happened. Just because a 5 year old says that an adult got on the bus and yelled at her does not necessarily mean that this is exactly what happened. I have had 3 kids and love them all very much, but their version of events isn't always particularly accurate. So, I would start with the bus driver. I would ask her what happened and I would ask that she separate the girls on the bus for the time being.
I would then make an effort to get into school and talk directly with the teacher. Make an appointment if it is necessary. Ask her what happened and what her impression is of the relationship between the girls.
It is crazy how much drama there is with 5 and 6 year old girls. My daughter is 7 and she does a pretty good job of staying out of it, but the day to day who is speaking with whom and who is friends with whom is crazy. Earlier this year we had an incident with my daughter and one of her good friends. One day they were crazy angry with each other and it escalated to the point where I asked the bus driver to separate them for a few days. Only a couple of days later my daughter asked hers to be one of only a couple invited to sleep over for my daughter's birthday. Little girl drama comes and goes quickly.
Do you know this other girl's mom? Can you speak to her? If you know her well enough to hold a reasonable conversation with her, I would consider having a conversation with her.