I wrote a big post and it disappeared so will keep it short. Probably not a popular view. Both DH and I have various law enforcement backgrounds so we use our experience in that to teach "reasonable force necessary to remove themselves from the situation". If they need to hit or push in order to get away, they will not get in trouble for it but if they do it out of aggression, it's not okay.
There is a child up the road who is a bit older than one of my children, who has autism. And he is an outright bully. My son isn't able to speak well as it is, but even so, the boy doesn't let up from being told to stop. Even when we parents tell him to (not including the mother who refuses to acknowledge her Angel could possibly be at fault). We have told our son that of the boy is on top of him, he is allowed to push him off. I don't expect any of my children to just grin and bear it, but I won't accept them being aggressive either. Hence why we try to teach them that they can only use it when absolutely necessary for their safety, so they can get away to tell an adult.