Get the number of the bus and call dispatch to complain. Tell them where and when it happened and the driver will be written up for it.
My district has a system called "Zonar" on each bus. Among other things, it stores data about speed, time and what lights are on. If I blew a stopsign and you reported me, my boss would look it up and verify I messed up and I would be in deep doo-doo.
The other feature I really like is it records the time that I arrive at a stop, how long I wait, and what time I pull away. The parents who come out late and say I came by early and left little Johnny behind while sticking out my tongue, they call the parent out on it and tell them to get outside on time.