Why do you think the lower sets go at a slower pace?
Because of confidence crash, because gaps were allowed to open and not bridged, because basics were not properly retained before moving onto to the next topic. Because they were segregated into lower expectations where they are in a twilight of 'working towards', but interventions stop short of 'working at', so they can't regain their solid footing, feel shaky, incapable, unworthy, abandoned, and they react to it. In short, because of sets and because the system believes they are not worthy.
I know, you want to say it is because they are thick. No, they are not. I am talking about those with average ability and children with average ability should be, and are able to get good outcomes in other countries. In France children with average ability pass BAC, 90% in a school year, 80% in each year of birth.
Ability is distributed equally in all countries, Brits are not more stupid, so they too should be able to pass Bac with the correct organisation of education. You need to remove barriers for 90% of brits to pass the level of knowledge equivalent to Bac. I keep repeating this because it is really important to anshor the discussion to some factual reality, as opposed to opinion elevated to religeous dogma that 35% of population are too thick and anworthy. This dogma comes from 1870, it's time to forget it. If the French can design a system where 90% of school year can do enough maths to pass Bac, than Brits should be able too to learn those maths, and the system need to be made to move and fit around this goal.
Other countries qualifications and outcomes are scrutinised by experts under Bologna process and are recognised as equivalent in content and outcomes. Basically the Bac I am talking about is equivalent to A-level grade C. This means the 10%-tile Finns and French who pass those qualifications actually achieve an absolute level of knowledge equivalent to grade 4 here. Actually the French achieved 8 out of 20 to pass. They didn't pass because of lower norm reference, they don't have norm references, they actually answered questions 35% of Brits can't answer. I repeat that because it is an important anchor of reality. Average kids in all countries can achieve better. In this country they don't achieve not because they are stupid, but because the system is deliberately designed to abandon them. Yes, you need significant change of the system, but you need to teach them actually to solve those equations. It is a matter of how to organise tuition to improve the absolute standard attained.
It is most certainly not a matter of moving the norm reference to fit the same current low outcome. It is a matter to teach differently and better, to improve the outcome for at least 90% of population. I don't know why everybody pretend to not understand this. Totally invested in the sacred immobility of the system?