Apologies for my wonky post. I didn't use my glasses! However, the problem with the Michael Morpurgo example is that he had the fall-back position of a public school. He didn't go to a secondary modern.
I am not sure what you are trying to prove, sandyholme. Most comprehensives lose children to the private sector who are bright. Many comprehensives have cohorts of lower achieving children that are 20% or bigger. Some will have the lower level of middle achieving children who struggle to get the GCSEs with Maths and English. You really need to look at the value added and not attainment results.
Regarding funding, the big difference between schools is now pupil premium funding and largely this is not going to the grammar schools. Where I live it is the secondary moderns that are the recipients and the grammar schools are howling. The other problem is that the amount spent per capita in free schools is much higher than in academy converter or LA schools. Generally they start with few pupils so their costs per pupil are high if they are to deliver a broad and balanced curriculum. The money they get reduces the amounts given to the LA for the other schools. If a new grammar was a free school, it almost certainly would get more money per child but little PP money.
Most teachers, in most schools can teach all year groups and all abilities. The skill of the Head of Dept and the SLT is to get the right teacher for each set. Teachers with patience tend to like the lower achievers and those that want to crack on and prepare children for the best universities will teach the higher sets. There is a shortage of maths teachers for everyone so people have to teach "best fit".
In my LA, the biggest challenge is to make the secondary moderns outstanding. Only one of them is at the moment whereas 12 out of 13 grammars are. One big problem faced by the RI secondary modern schools, of which there are a few and 3 in one town, is that there are too few good teachers and learning suffers. No such problems in the grammar schools. This is not necessarily a secondary modern vs grammar school issue but it is a more attractive proposition to teach in an outstanding school with brilliant leadership than in one that is just so so.