I know exactly what you are experiencing right now Martin.
Our youngest recently defended a schoolfriend (who gets picked on a lot because of his mental health disabilities) who was being beaten by a much bigger child, but because he used his martial arts skills to defend his friend we were told that this was considered "wholly & completely unacceptable" behaviour.
The entire incident was caught on CCTV (which we were shown), and it clearly showed the bully being hit once and once only, and our son then stepping back (which is what the Sabonim teaches) and stopping at that point.
The bully then got up and went to attack our son, at which point he promptly hit the bully with the same "sucker punch" again (the bully didn't go back for 3rd's after that one).
Needless to say both myself & my other half were not impressed, and we made it quite clear to the headteacher that we fully supported the way our son dealt with the situation and saw no wrongdoing whatsoever.
All that achieved however was for us to be told that if our son does one more thing then he will be excluded permanently, and the school also reported us to social services as they considered we needed to go on a "parenting skills" course because we made it clear we would not budge on our stance.
IMO the world has gone plain f*****g mad, and if we don't do something about it shortly (eg, go back to the way it was done in our day), then I don't hold out much hope for the human race