I guess they get paid an hourly rate. That would vary on the bus company that they work for.
I am sure it varies with each school district. My brother is a coach and is licensed and certified to drive a school bus. If he drove the team to a sporting event he made $50. I know it is not a field trip, but I figure it is close enough.
I have also lived places where the bus drivers were not employees of the school district. The district had contracted out the buses and drivers. I am sure they would either get paid by the hour and my guess would be it would be time and half because it would be over their normal time for their routes.