what you do when 24 children got the maths, are ready to move on and you have 6 who need the lesson again
You start from the principle that all children can achieve in maths.
They probably didn’t understand and were left behind on a few preceding, more basics concepts. What you do is you don’t let that happen from Reception onwards.
You change the methodology that underpins the maths teaching, from rote learning repetitive narrow examples, which some learning styles may find meaningless, to learning fundamental mathematical concepts in different context. You teach them to think, not to do exercises. You respect their learning style, people learned sitting in rows for centuries and turned out all right.
You teach them to derive satisfaction from learning new ideas. New ideas from the teacher at the front are more interesting than the rubbish joke from the disruptive attention seekeing peer at "the bottom table". Teach them to respect themselves and others. Yes, let them explore , ask questions and find their own answers as part of the mix, but in a structured way, when they are ready.
The Mastery method is clear – you don’t wait the assessment to find out they are behind, but do rapid intervention when they are struggling, so they are ready to move on with the others. Yes, you work more in depth with the others and take time to teach the children that don’t get it the fundamentals until they really understand.
One lesson is a very short time in a 13 year school career. It is a lesson that lays foundation for their success in the following school learning and in their lives. They are worth it. Teaching them is what teachers are for.