I don’t understand why you think that higher academic ability translates to better behaviour?
I didn't say it did. I've taught plenty of well-behaved academically weaker students and plenty of academically able but poorly-behaved students. But far, far more poorly-behaved students who are below average ability. This is something any teacher (and and pupil) could confirm from everyday observation.
Selective schools have a different behaviour profile for a variety of reasons, related not only partly to the ability of their students, but also to the attitudes of the parents who chose to send their child to grammar school and the ethos of that kind of school. In a school where high achievement and good behaviour are seen to be the norm (by staff and pupils), good behaviour tends to persist. At my school, if a student swore at a teacher or was openly defiant or rude (commonplace in lots of schools), most of the rest of the students would be horrified.
Are you saying that children with lower academic ability are automatically disruptive? My DD is y11 and will get all 8s/9s in her GCSEs at her “comp” so just keen to understand your data source?
My data source is 27 years of teaching in a wide variety of schools. My children will get great results at their comp too, because they are very bright, motivated and have supportive, academic parents who know the system. What about the students who don't? There was an article in the papers today about the staggering amount of learning time lost to bad behaviour.
The fact that less able students are more often poorly-behaved isn't a direct correlation between intelligence and behaviour imo. It's that some low academic achievement is down to attitude rather than ability. And the fact that many students who have low academic ability become disengaged with learning because they are frustrated and don't see the point of behaving/working if they feel they still won't do well.