[stlnative]

Another possible reason why more than one shot may have been heard...

It is possible that if there was 2 bus drivers that one bus driver stays armed due to his type of work and maybe he shot off a round or 2 in the air or towards the woods to let the prep know he was going fight back if needed.

[yukong]

I don't believe for a second that the shooter fired off more than one shot. From what I can see on tv reports, there is a wooded area just to the right of the buses. Most likely, the sounds were reverberations, echos from the shots. Not uncommon. If there was more than one shot, then there would be evidence of that in the bus.