I can't speak to the legality of what you did but I tend to agree that it wasn't the best move. It actually does sound like passing to me because the point is that kids are supposed to be able to get off the bus and either cross in front or walk down the sidewalk. I'm always extra cautious in those situations and err on the side of stopping if I don't know what to do.
All that being said, I'd fight the ticket just in case. You don't want a moving violation, especially one for passing a bus, on your driving record. It can affect your insurance rates.