I realize that you may be looking for a cause of action against your employer (e.g., school district). However, as a general proposition, California employers are immune from employee claims of negligence based upon workplace injury, if the employer carries workers compensation insurance. This is true, even though you are assaulted during work -- unless you are assaulted by the employer's management personnel.
See Labor Code § 3601(a)(1); Arendell v. Auto Parts Club, Inc. (1994) 29 Cal.App.4th 1261, 1263-1266.
In theory, you could sue the child, and possibly recover damages against the child's parents. However, getting a jury to find in your favor, given the circumstances of the "big, bad bus driver vs. the poor special needs child," is simply a high-risk lawsuit, and I doubt many personal injury attorneys would be willing to take the case.
That said, if it were me, the next time I was attacked by this child, I would dial 911 and ask law enforcement to arrest the child, either for a willful assault or to be taken into custody for observation on a mental health observation hold. See Welf. & Inst. Code § 5150. That will get the county juvenile authorities involved, and take it out of the hands of the school district.
At least you would have protection from the child, if nothing else.
Hope this helps.
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