I'm sure there are many factors that will not be represented in a simple graph. Even with all the facts there would also be the question of cause and effect. it's never going to be as simple as x=y or Vernon's opinion.
It isn't my subject of expertise, I wouldn't know if the majority opinion expressed by the education authorities is any more valid than Vernon's. I suspect it's somewhere in between and a lot more complex than this thread will establish. It seems well established that covering a subject three times is effective, at all levels of education. It would therefore follow logic that to miss one would be a disadvantage and there should be an advantage to other pupils if a teachers time isn't spent helping a minority to catch up. I do think that expressing an opinion as fact and patronising those who question it as slow learners, along with the accusation that all head teachers are liers, is unlikely to improve anyone's understanding.