But is it, or is it teaching the highest and lowest ability children to their ability and the average middle ground child just plods along having the best of no worlds?
In a completely mixed class, we are told to 'teach to the middle' anyway so its completely taught around the middle ability and we extend or support/scaffold through targeted questions, task, and toolkits offered to for support for the highers and lowers.
This scenario is the hardest to teach in because you can't offer enough support to the lowest attaining children who often, by dint of their needs, don't actually use the support tools we provide - eg word mats, concrete resources - without adult assistance. Either because they forget they are there, forget how to use them correctly, forget when to use them or just can't be bothered with the effort required because its still difficult for them they just have extra 'things' on their table.
By the same token, we can't appropriately extend the highest attaining children because we can't get the focused time required to challenge their understanding and thinking because our attention is diverted because one child (usually a middle) hasn't quite got it or a lower is making guns put of cubes because it's more fun than using them to work out arrays. And they can't quite remember what an array is in the first place.
Setting them means that the middles, far from being overlooked, are actually able to learn because the pace is better suited to them.
It also means you can set the middles who have got it off on their task independently and invite those who still aren't sure to stay with you for a bit longer.
The middles are more likely to be 'plodding' when classes aren't set.
The range of ability across the board is actually quite small when you've removed the highest and lowest from the situation.
I don't know why some people assume we're utterly incompetent.