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If your state employs union bus drivers, there is absolutely nothing you can do that will correct this situation within the normal complaints channels.
There will be no corrective counseling and/or action taken against the driver.
The situation will not improve, and will escalate in a negative way against your child, whether or not your child is at fault.
Even if several other parents have similar complaints against the driver, do not expect them to take any action at all.
They will not.
Transport your child yourself.
If you want, continue to press the issue with the taxpayer funded, supposedly professional adults, in the transportation management department, and at the school, while your child is "safely" away from direct physical repercussions, but know the driver, and the administrators at the elementary school will encourage your child's peers to torment him for your actions.
Resolve the entire issue by secretly contacting a legally ambiguous communications with a third party mediator.

Or you can just shut up and take it.