You could be cited for a marked lanes violation and for failure to stop for a school bus and perhaps negligent operation. The marked lanes violation and failure to stop citations are civil motor vehicle infractions and not criminal, but negligent operation is criminal. I realize that you stated that the lights on the bus were not flashing and the stop sign placard was not extended, but in a situation where you are being reported to the police, it is quite likely that the witnesses will say that you passed the bus while the stop sign was out and the lights were flashing.
There may be an identification defense if you are cited or charged, assuming that somebody got your plate number. Under no circumstances should you make any statement at all to the police; the police are only there to collect evidence to use against you and if they are asking you questions, you are a suspect and nothing you say will convince them you did not commit the crime. If you receive a citation in the mail or any other documentation from the police or the courts, contact an attorney immediately.