Investigation against school associate

One week ago, I got a note sent home from my son's school informing me that he called a teacher an "Old, Ugly Bitch." Do not worry, that vocabulary was learned prior to him moving into our home but it is however something that we have been battling for well over a year. When speaking with my son about the inappropriate verbiage he explained to me that the teacher would not let him play. He did not fully comprehend what happened but all that he got out of the incident was the teacher was being mean and preventing him from enjoying the slides. Two days after I got the note sent home, I got a call from the school administrator informing me that an incident occurred that was being investigated. She said that 3 parents witnessed and reported a teacher grabbing and screaming at my son. I went up to his school and had a discussion with his classroom teacher to get all of the details. She said that the person who was involved in the incident was an associate, not an actual teacher. She also explained that they had previous problems with this particular associate so she is quite regularly watched closely. Absurd. Internally I wanted to lose it... someone who is unable to handle a child with behavior problems should NOT be working with preschoolers!!! My son's teacher advised me that she witnessed the associate pinning my child against a gate and was screaming at him. So she pulled my son away from the associate and got him redirected. She also informed me that she watched a video with the associate and explained to her what she did wrong. Then she said I would be contacted when they knew more.... Well today, I got a call asking if me and my son could come into the office for an interview for the investigation. My son does not like to approach feelings so when the investigator asked him questions he avoided them. I attempted to prod questions from him but again he held back. He probably doesn't even remember the incident to be honest. He is very smart and has an amazing memory but quite frankly, he does not have the cognitive ability to talk about memories. Low and behold, the investigator informed me that not only was my child held against his will next to the fence, he was also grabbed and dragged out of a slide. Then pinned against the fence and screamed at. NO FREAKING WONDER my child has some behavioral issues!!! If this is how things are going at that school then this absolutely explains everything! I am so pissed. I was not angry...when I knew that there was a chance this teacher would be punished...however this afternoon the investigator called me back to inform me the case was unfounded so she is not getting child abuse charges....simply because I FOUND NO MARKS! What about the trauma he may have experienced? My kid has been through enough and school NEEDS to be a safe place for him!!