Yes, Boneyback, my DD has just finished Y9, and she is the first cohort that HAS to stay in some form of Education until 18. I would love to sit in on an A-level course where bright but severely dyslexic DC's that can't READ the textbooks and need a scribe to write for them, and watch an unqualified teacher have to differentiate an A-level lesson for them...
And to answer your question about Academy Grammars, well, their pupil population is hardly representative of the normal 'Comprehensive' intake, is it? Their intake is made up of the however many percent most academic DC's, that WANT to be there, and have chosen to sit entrance exams to do so.
Not quite the same as in a Comprehensive, where there can be an attainment spread of 3-4 years or more EITHER SIDE of the average for that year group. In my DD's bottom set Maths group for example, there are DC's working on NC level one, right up to NC level 4medium. DD is just about hitting NC level 4high, so is being moved up to the middle set. Which will cover NC levels from 4high to level 7low. Anyone working above level 7low goes into top set.
Could an unqualified teacher effectively differentiate for the spread of levels within a class? Could they control the high levels of DC's with both behavioural issues AND pretty severe SEN that are all mixed up in the bottom set? Could they cope with helping to explain a method of working out a sum to a DC with Dyscalculia at the same time as dealing with a DC that is calling him a cunt, at the same time as stopping the DC with Autism from having a meltdown, and breaking up a fight in the classroom, while keeping the rest of the class engaged and learning?
I bloody doubt it.
The situation in Private schools, or a Grammar Academy is somewhat different to the reality of a Comprehensive school that has converted to an Academy.