This is pretty weird.
The main law for school damage other than criminal damage is negligence, and if my dd caused damage by negligence I would have to pay as she is a minor. I watch a lot of Judge Judy.
By the way Alice I think they would have to prove criminal intent which would be incredibly hard of your dc is in a special school and does not realise that he is doing anything wrong.
But when it is a matter of poor supervision, particularly of ss children, then the school should be liable. They might want to review their insurance.
If it is reasonably predictable that dc will have meltdowns and lash out when stressed, then their policies should a) try to prevent that occurring b) protect dc and staff members when it does and c) protect the fabric of the building.
I really struggle to see how they can enforce their policy through a court.
So what to do about it?
Well I would ask for a copy of the policy, and then see if IPSEA's lawyers would look at it and sort it out with the school/LA legal bods. Try to keep out of it if you can. Your nerves sound stretched enough to me.
If IPSEA doesn't run with it, then pm me. There are some pro bono solicitors/barristers on a register who might be able to give an opinion.