In a primary school in Essex, 30 yrs ago, we grouped Y5-6 together for English, and taught in two sets. I taught the lower set, and Y6 teacher the higher. I focused on reading and spelling difficulties that the other set did not have, and the Y6 teacher on more advanced work that my pupils couldn't do because of their difficulties. No more than a small experiment, but highly satisfactory to all concerned.
Children for whom learning is not straightforward need work closely matched to their learning needs, or they can't understand it, and so make little progress. They also become frustrated. It's no co-incidence that behaviour problems predominantly affect lower-attaining pupils and those with SEND, as they are getting no satisfaction from their work, and are frequently faced with a daily round of failure. Attempts in Essex, by the primary inspectors, to replace literacy as the basis of education with first-hand experience, simply led to lower standards of literacy.
The kind of streaming that was prevalent at the time of the Plowden report in the 60s - over 90% of primary schools streamed - has been replaced by a mixed ability dogma that is putting huge strain on teachers, and is meeting no-one's needs. An alternative is in Learning without Limits' approach to maths. Classes offer different levels of difficulty/challenge, and pupils are allowed to choose. This should be developed. As people may know, Dame Alison Peacock, head of the school that used this approach, is CEO of the Chartered College of Teaching, of which I am a Fellow.