I think violence is acceptable in certain situations. If the need arose, I would hope my kids would be violent. That's one of the reasons they learned kick-boxing - so they stood more of a chance of their violence being effective rather than ineffective.
Self defence is OK. If someone attacked me I'd bash them with my handbag!
Owlswing, I was thinking over what you were saying about menacing schoolchildren. I experienced that some years ago, on two occasions, on neither of which did I have my car. Once I picked up my cousin's boy from school, he was about seven, and we walked around the corner to the 'bus stop. A lot of older boys from his school also arrived en masse at the 'bus stop and when the 'bus came along they surged towards it, regardless of people who had been waiting there before them, and pushed their way onto it. One or two of the people waiting there said it happened like that regularly. They had absolutely no consideration.
Anyway I heard the headteacher received complaints and actually came out to spy, lurking in a shop doorway, then he blasted them in assembly. Quite right too.
The other time I had been to see someone in hospital and went by 'bus because of difficulty parking. On the way home the bus stopped opposite a school and a lot of kids got on. A bit further along it stopped outside a girls' school (locally known as St Trinians), quite a few girls boarded the bus and proceeded to attack the kids from the other school with water bombs and eggs! The bus was a mess, even the driver was egged. He stopped the bus and said he was fed up with it happening nearly every day! It was quite shocking. I don't know what happened after that - the school is different now, amalgamated with another school and has a lot of Chinese boarders who are very well behaved.
I think teachers, especially department heads and head teachers, as well as parents, should be proactive in ensuring better behaviour. Sometimes kids get carried away in a crowd, they just don't think, don't realise how intimidating they can be.