My boy was bullied for 2 years and kept getting detention for standing up for himself. "Raising your hands" is not allowed. They defo try and ignore bullying despite all the wonderful words. In the end we had a "come to Jesus" meeting with his form teacher and the Deputy Head. The teacher just stared balefully at us for an hour and intimated that his "personality" and "behaviour" was the real issue. At that point we moved him to another school where he made a number of very good friends within a few days and was very happy until he left last year. All of this was at Forest School in Horsham which, supposedly, is very strict on bullying. My personal experience is that the school was paranoid about admitting that any bullying takes place and they are very happy to talk at length about their anti bullying policy but it is not effectively implemented
I know that this is an emotive subject, but I do have some experience of dealing with children's behaviour whilst managing parents reactions and expectations.
Again, his reaction to your interpretation of bullying seemingly is to hit out at other pupils and the school then put your son repeatedly in detention, it offers an insight into those incidents.
I would guess that the parents of those getting hit by your son, might have a different interpretation of events and the school who are the intermediary's concluded that your son was at fault.
Sometimes parents cannot accept that their children's behaviour is below expectation's and excuse their child's behaviour whilst blaming everyone else, bullying has become a favoured tool for excuse. ( I understand the seriousness of genuine bullying )
I am glad that your son had happier experiences up until he left, but maybe he just matured or modified those behaviour traits that got him into trouble in his earlier school life.