Of course some children are below average in ters of ability. Just like some are average and some above average. That's how averages work after all.
But dyslexia, dyscalcula and other things like that do indeed exist and with the right intervention early on, which can be within lessons or external to lessons, it means children with those issues can learn to work around them, learn strategies and can then reach their full potential within lessons.
Dyslexia and fyscalcua have nothing to do with academic ability. There are many very academically able dyslexics around who have been shown ways to overcome their difficulties. Should they be prevented from doing so?
Similar children who are below average can be given additional support in order to raise their levels in key areas such as phonics and numeracy. By doing this and giving them a bi of extra help - again either external to class or within the class - they can pick up the bits they are struggling with, make their own progress and become more able to access the maintream lesson.
Also there are some children who are above average, possibly even G&T. And again, the should get some extra support to extend and challenge them so they too can reach their potential. Again this coud take place in class or external to class. Should we stop this too?
And what about those who are average? Do we just tailor everything to them? Or should we aim even higher and put in some support here and there to raise their game too?