The difference isn't how students are taught (in a lot of cases) instead it is students ability to retain and process information.
That difference is relatively small in 'normal' population.
If there are significant differences, it is a child with SEN. For these children, their needs must be met. Currently, especially with cuts, the SEN system is like a fortress that you need to siege and battle for years before getting in. Just a taste here.. In this time DC with SEN suffer, lose confidence, develop anxieties, psychological problems, get behind.
SEN does not equate to low ability at all, they just have specific barriers that need to be addressed.
Poor outcomes are always excused by 'low ability', but genuinely low ability children are just 2%-10% of population, depending where you draw the line. Nobody argues they should have GCSEs grade 9.
Average is 90-109 IQ and starts at 25%-tile.
Within those 35% who don't get good GCSE Eng&Maths, 15% are in below average (80-89) and 10%, i.e. a third, are within average range of ability IQ>=90.
Some like to draw the line at IQ 85, that is 15%-tile, then 20% within the 35% would be within average ability range.
DC with the whole range of good ability from low average IQ>= 80 to very superior IQ>= 130 can have SEN. If the needs were really met timely and properly, these children would progress at broadly the same rate. But yes, they do cascade down to bottom set because of lack of provisions and because some cynical schools equate SEN with low ability.
The bell curve is the same in all countries, yet in other countries get better educational outcomes for 80% or more of their population without sets. It is not the ability, it is the system that is the problem and sets are exactly the instrument that drags down the average and below average children, locks them out of good outcomes.
They can't climb out because they are not taught the same material.
Some children are in secondary modern since Reception, those on the lower 'tables', the parents just don't realise that.