OP, you do realise that sending your child private will not ensure this doesn't happen, right?! Just because a child's parents can afford private does not mean they cannot have similar issues or SEN which leads to violent outbursts?
In fact, in my experience, especially now with the VAT issue, private schools will turn a blind eye as they need the fees.
What private schools do have is more resource to support these children, that is absolutely correct but that is not a guarantee that it won't happen. The aim will always be to keep them in the school community, in class.
You sound incredibly emotional, and I understand that, but the things you describe are sadly not new or unique and will be found in every setting with children. Before you react any further with governors letters and reports to the council and moving to private school, why not take the weekend to reflect.
You also don't mention what your child wants. Are they feeling threatened or are you telling them that? I have children of a similar age and they are generally very understanding of the SEN kids and issues because they educate them about it - much, much more so than when I was at school. Moving a child from friendship groups and a familiar, positive setting is a huge gamble. Many parents often underestimate the longer term impact on the child.
I.wholeheartedly agree with the majority of other posters in thY you should be asking the school what they are doing to safeguard your child, that should be the focus.