Yoo-yoo! Get off the ledge, Molly!
I knew a school bus driver once. I don't know how she survived her work. Mamas who think they have a tough time managing their children at home should drive a school bus for one day.
I'm assuming that this driver is a worthwhile member of the school staff and totally trustworthy. The driver is just trying to get her job done. Her job is to get the children to and from school on time with no problems, no injuries, and no grief from her employers. She's not looking at individual children - she's looking at a teeming, noisy mass of something or other.
So teach your little girl that bus drivers sometimes yell (at her, even!) but that doesn't mean they're nasty. Is there a way she can help the bus driver? Yes - she can help by learning the rules and being kind.
You could even do this (are you ready for it?): teach your daughter to get on the bus and say "Good morning, Mrs. Smith" to the driver. (Saying the driver's name is important.) She should say it every day, even if the driver doesn't respond. Let your daughter report to you how it goes every day.
In the afternoons, she can learn to say (at least for the next couple of weeks), "Mrs. Smith, I'm not sure yet where I get off. Would you please tell me when we're close to Adams Road?"... and, when she's ready to disembark, "Thank you, Mrs. Smith. Goodbye."
Think of this circumstance as a terrific opportunity for your daughter to learn good manners toward a grownup in a way a six-year-old can handle by herself. Let her have that chance. If she's a girl who talks to her mama about things, you can keep up with the adventure and be there for any road bumps along the way (kids generally have less trouble from drivers than they do from other riders).
Wait to pull out the Mama Bear suit until it's a necessity. Too much Bear too many times dilutes its effect.