Can I prove school's negligence?

3 yrs ago, school failed to suspend bully for pulling a needle as a weapon pulled on my child. I told them of bullying that went on for 3 months with no action taken. After that, my son was attacked, eye socket almost broken. I just learned from the school district that they were never told about it. My child's school never notified the school district of the attack that even went to court. Now my child suffers PTSD, depression and anxiety. I feel if bully would have been dealt with regarding the needle, the attack would not have taken place because "zero tolerance" should have gotten him suspended/expelled for the "weapon" (district admits they should have and that it was indeed considered a weapon) and admit they failed here. Now I incur medical costs from counseling, meds, hospitalization because of the attack causing him PTSD, but they totally swept it under the rug. I know there is statute of limitations, but who holds the school accountable for their negligence?