Agree to a point Nick, but , my wife is a headteacher in a primary school in a very rough area of Barnsley, and, this attitude of 'We can't do anything with the little gits' just doesn't wash with her. Her school was left to it's own devices for 30 odd years because no one dared to challenge the culture that had evolved over that time. She operates a zero tolerance policy with reguards to bullying, by pupils, parents or, indeed, staff.
If all primary schools were run this way then it wouldn't be half as bad in secondary schools. The rules of engagement need to be set at a very early age.
Excellent to hear that your wife operates a zero-tolerance policy. Should be mandatory in all schools.

To clarify my point a bit (and I'm talking secondary schools now), I mean that the range of sanctions a teacher can apply are somewhat limited. Detentions, inclusions, temporary exclusions then finally permanent exclusion. At that point, the parent normally appeals and either tries to get the exclusion blocked on a technicality, or swears blind that the School has got it in for their little darling. You may say, well these are not limited sanctions, but they are when you think about it. The kid who causes low-level trouble in school often knows how far he/she can push it without the teacher resorting to a formal sanction. And, of course, if teacher of unruly pupils hands out too many detentions, it can reflect badly on him/her. ("Can't control the kids, that one..."). In the old days they would get some sort "clip round the ear" or made to stand in the corner. And, when it comes to detentions, well, if they skip a few what happens then? A few days off on a temporary exclusion a few weeks after the original offence. Of course, detentions and exclusions mean little to the habitual rule breaker.
Fortunately, the school in which I serve as a governor (1600+ role of 11-18 years) has a head who also operates a zero-tolerance policy and will not tolerate any bad behaviour. However, in any school, big or small, it is the everyday teacher who faces the frontline, with a minority of disrespectful pupils who enjoy disrupting the learning process for the majority. Some teachers can actually be bullied by pupils and, at the end of the day, live in permanent fear of being falsely accused of intimidation or physical/sexual assault. The kids know that and they know that the burden of proof lies on the teacher's shoulders.
So, while there ARE sanctions available, teachers are no longer permitted to resort to the
instant justice of the old days (clip round the ear, stand in the corner, etc.). These days it's, "Smith, you've just got yourself a detention". "Who cares Miss? Stick it up your b*m" General hilarity amongst the others and Smith earns himself a bit more playground respect.
It's a culture thing, I'm talking about, (or, maybe a "lack of culture" thing).

Also agree 100% with the comments about the parents. It all starts at home.