Camilla Cavendish said on Question Time last night that Grammar Schools were autonomous and disciplined. In my 1950s-60s grammar school there were riots in almost every Chemistry lesson as a consequence of an eccentric teacher who was there for years. The school was broken into one night by some of the boys in my year and they caused quite a lot of damage. I was involved in various milder forms of disruption across two years that was continuous. I'm not proud of the fact but in terms of 'discipline' was in those terms a disaster. The school didn't know how to handle these matters.
Autonomous? County grammar schools came under the aegis of the Local Education Authority. They were no more or less autonomous than the other schools in any given area. It's possible that the prestigious ones could throw their weight around at meetings more than the Secondary Moderns but they weren't 'autonomous'.
There were 'Direct Grant' grammar schools which were forerunners to Academies. These were usually old 'foundation' schools which were funded directly from central government were outside of Local Education Authority control. There were also 'Independent Day Schools' which were private but which accepted some children at 11 (usually) on scholarships. These were independent and even more 'autonomous'.
Beware people on TV talking about the history of education. People tend to make up stuff or just pass on oral legends.