Hiddeninplainsight, no child with "mild learning disability" would have a statement, let alone their own learning support assistant (at least, not one with bespoke funding) although obviously the school/teacher may use their discretion to allocate support/intervention time to them to close the gap. Similarly, many schools will flag more able children and will differentiate for them -- it just may not be as obvious.
I have a lot of sympathy with your frustration with insufficient stretch and challenge at the very high end but I would say that moving high-achievers on faster through the curriculum is a bad solution.
Children are usually keen to proceed because it brings prestige, teachers often quite like it because it's how children in the past have been "stretched" and parents think it represents progress. But it doesn't generally bring challenge because most content is procedural quite literally, a list of things to know how to do and high achieving kids take to procedures quickly. And sure, they could be working two or three years ahead but it's the illusion of challenge and, by widening the gap, you're making it impossible to keep the class together and fostering exactly the kind of untenable situation you describe with children working on objectives years apart within a single class.
Take an objective of using the digital root (the sum of the digits of a number) to identify multiples of 3. For children who get this quickly, later objectives which follow could include simplifying fractions using this skill, perhaps even adding and subtracting fractions.
Or you could ask them to explain to the class why the digital root identifies powers of 3 but not powers of 4 and to use this to predict and explain which other multiples between 1 and 10 can also be identified by finding the digital root. And if they have that, can they use the operator blocks and If/Then/Else to create a program in Scratch which identifies the digital root of any given number?
The former pushes them through the curriculum faster but only utilises the same kinds of skill. The latter doesn't involve any new curriculum content but does involve broader and more challenging application of the same content through reasoning, hypothesising, explaining, proving and applying -- skills which will be infinitely more useful later on and will ultimately make them much stronger mathematicians.