The bus driver is the witness, and technical complainant.
At times the running of a light is subjective in nature. If a non professional witnesses it, as opposed to an officer on scene and knowing the law in detail, the "weight of the evidence" is probably in your favor, IMO. Unless the violation was so clear cut, there is no room for subjectiveness.
This is not an assured aquittal, by any means.
Additionally, it is possible the bus driver was at fault and is making you a scapegoat. The way I have personally seen some school bus driver's handle the road, it is a wonder more bus accidents are not reported.
If you are convinced you substantially complied with the law, stand your ground.

