The emergency exit is supposed to have an alarm. Why didn't the driver notice? why didn't other kids on the bus say something? Why didn't the driver notice the sudden change in air pressure and noise in the bus? While the driver can't be blamed for the girl getting the door open, he sure as hell can be blamed for being oblivious to it.
It's a good thing that EMT was where he was , the girl may be dead now.
When I was a kid, I had the same thing happen, an emergency exit came open, alarms went off, kids were yelling and the driver just kept driving. luckily nobody fell out.