I think there is a lot of misperception on this thread about what is happening in schools re. ability grouping. It is not that different levels of learning have been removed, just that there is a move away from fixed ability grouping towards flexible grouping - ie you no longer have "circle group" that always sits on a particular table and does an easier piece of work than everyone else, or "hexagon group" that sits on a separate table every day and does harder work. Grouping is decided on a more or less daily basis, or sometimes changed within a lesson, depending on how pupils are demonstrating their understanding of that particular piece of learning.
Different levels of work still exist, it's just in a less rigid way. For example, if pupils are unable to add 3 digit numbers, some pupils may always use physical manipulatives to support them, or work with adult support or do all of the above with smaller numbers. For a reading activity, children may read a text in mixed pairs, but then some children will move to work with an adult, some may be given a more challenging set of questions etc. It is not often successful to pair a very able child with a very low-ability child, but mixed pairings of say a slightly more able child with a child that is working a little below average can work well.
Challenge activities are always provided but are available to any child to attempt. They are not solely given to "hexagon group" to complete.
Teachers decide who requires support during the lesson. It may be that a child you would have thought needed support performs well so you send them to work independently, but you notice 2 children that could do similar work the day before are struggling so you bring them to work with you, whereas back in the day you had 5 groups all doing a slightly different activity and the teacher remained with one "focus group" each lesson. When you mark books, you decide if there is a group of children that needs further review in the lesson the next day while you start everybody else of on the new piece of learning.
Children with SEND are always provided with work at their particular level of need. It may be that they can cope with an adapted level of what the rest of the class is doing, or they may need to be focussing on something completely different or it may be a mix of the two.
It is absolutely not appropriate to teach every member of class exactly the same thing and have some children who always achieve easily and some children who never understand.
Having taught both ways, I can see a lot of positives from the move away from ability tables. It prevents pupils from being pigeon-holed. There are still times when ability groups may be appropriate, for example in guided reading where it is best to group together children that can access a similar level of text, but on the whole flexible grouping provides a lot more freedom and, IMO, better suits individual children's needs.