The school has no liability for a car parked in the car park. They will have many disclaimers regarding this.
However,
They will have a duty of care to the child with special needs (and in fact, any and every child) and as such the question is whether they were negligent in their supervision of the child that led to the child causing damage to the car, intentional or not.
If it was my car I would be getting my insurance company to contact the school with this particular line of questioning/claim.