What exactly do you mean by telling the boy to go to the back of the line because he can't budge?
First, try to figure out exactly what she means by spit on her. Ask her to demonstrate. If this boy has to go first, your daughter needs to know that, and she shouldn't be bossing people around. Also, if this other girl is mean to her, she needs to let you know. I think she did good by not continuing to engage this other girl and telling you about it.
To take it further, which I don't think I would do right now, you could call the Department of Transportation and ask them to clarify the situation with this boy or if the bus driver saw what happened. If any other parents wait at the bus stop, see if they saw anything. You could contact the school or her teacher and see if they can guide you, help figure out exactly what happened, or contact the other parents. Again, I don't think I would leap to this step immediately, but I have contacted the school over an issue of the kids when walking home, even though it was off campus. They take the safety of the kids very seriously from door to door, so if there's a big major problem, they can probably help you resolve it.