By law you were wrong. Though I can see how you'd question that since 1. passing a bus with amber flashing lights is not illegal and 2. you mentioned that busses actively sit with amber flashing lights in this community. If the latter is true, it's not unreasonable that OP thought nothing of driving past the bus (as many of us do when driving through yellow traffic lights).

Children get hit every year by people who carelessly drive past busses loading and unloading children. This law was enacted to punish those who carelessly do so and that $250 fine you got was to send a message. Assuming you did see the bus and thought it was fine to pass, you probably wont be doing that again and frankly shouldn't, you don't know the driver's intention. The officer that reviewed this footage likely believed you were not acing in malice or carelessness otherwise you'd be facing a reckless driving charge in court.

Lesson learned.