IMO, there is plenty of blame to go around in your situation
The sub needs to be more careful when loading the children on the buses - in my experience, the other kids are usu. trying to tell the adults that it's not the right bus. The drivers maybe should be given an assignment sheet that informs them if a child is absent, or has a note to ride a different bus so that THEY know when something is amiss.
My disabled daughter had the same thing happen when she was in the primary grades. What saved her was that she refused to get off at the wrong stop (in a rural area, next to the major highway, with the nearest house 1/4 mile away); the driver radioed to the school office that he had a child that refused to get off and was hanging on tight to the seat, and he couldn't understand what she was saying - he added that she was blonde but was speaking Chinese! (By then, I had run to the school, after calling them that she didn't get off at the stop, and the other kids kept saying that she was on the wrong bus and crying cuz the sub-special ed teacher forced her onto the wrong bus).
To overcome the bad experience, I made sure that she knew the number of her bus, and the street sign. I enlisted the help of the other kids at the stop the next morning and we built a small tower of rocks and sticks to designate the right bus stop.
I know this situation happens too frequently - it happened several years later to a 7yo, and wasn't found out till my girlfriend, her kids & I drove by a crying child, and the oldest son yelled to us that she was in his class and that something must be wrong!! So I immediately made a U-turn, and by then a lady in a nearby condo had come out to see what the commotion was about, so we were able to use her phone to call the school, who sent out another bus to pick the child up. Later that night, the girl's father called to thank me - that poor child was so upset, she had peed her pants
As for my daugther, I had already taught her that she should not get off any stop that she didn't recognize - that they would call me if that happened, and I would come down and get her. The principal did a thorough follow-up of the situation and let me know that the driver was getting cited for trying to pry my daughter's fingers off the bus frame.
In the second incident, it was the director of the bus division that forced the other child off the bus (according to the other kids that were there that day); I wrote a letter to the school. After that, when the head driver drove my daughter's bus, she was just generally mean-spirited to my kids. I warned her that I could take her nasty attitude, but if her behavior ever led to any misdeeds, I'd make sure that she was disciplined. I did speak to the principal about her attitude, so he was aware of this.
children have enough to deal with in school - the last thing they need is to be let down by the adults that are hired to protect and guide them!