Passing a stopping school bus with yellow lights flashing is not illegal. Passing a completely stopped school bus, with it's "stop arm" extended is a violation. Even if you committed an infraction, unless there was a witness who can identify you or your vehicle (e.g., license plate), it's unlikely you will get a ticket.
This information is provided for general purposes only and should not be construed as legal advice. No attorney-client relationship is created with the furnishing of this advice. Attorney licensed in North Carolina only.
NCGS 20-217 makes it unlawful to pass a school bus that is displaying its mechanical stop signal or flashing red lights and the bus is stopped for the purpose of picking up or dropping off children. If the bus was "stopping" and not "stopped" when you passed you would not have violated this provision. In any event, to prosecute you for this violation, the state would need a witness or some other proof that the violation occurred.
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Traffic tickets are normally given at the scene after a law enforcement official observes the violation. Since that evidently didn't happen it is highly unlikely you would receive a citation at any later time.