I am pleased to hear this. I don't believe intelligence is fixed.
People love to categorise children, label them, put them in boxes etc. Problem is labels can be very sticky to peel off. A 'slow' child that goes on to do very well is often credited with a surge of effort, an excuse is often found for a 'bright' child that seems to regress & not 'fulfill their potential' - getting in with the wrong crowd, home circumstances etc.
Truth be told perhaps things weren't so black and white in the first place?
I know a retired primary school teacher who tells me she could 'always tell' who the 'bright' ones were in reception. It's a subject that really interests me and I've commented on at length on here. In many schools ability groupings seem to be quite stagnant (not all I know). How many have a classroom situation where more than 1 or 2 children are moved every term or so? I am interested in what's unwittingly messaged to children in the classroom and how many essentially stay in similar ability groupings throughout primary school. I know of children who were/are actually much more capable than was first thought and have self esteem shot to pieces because no one picked up on their potential until they were much older. Similarly, I know of children who were seen and treated as being extremely bright (great handwriting early on, early readers etc) and put under pressure etc.
I read with interest on another thread, competitive parents? that a capable child was moved down to a low set - probably aged around 11 plus - because other parents were pushing for their child to be moved up. The parents of the capable child were too busy to be that involved with the politics at school etc to realise what was going on.
If groups are fluid, flexible etc then there should be no problem. I believe intelligence is learnable to a degree and minds expandable. If students are treated over an extended period of time as if they are intelligent they become so. (Unless there are learning impairments).
It's human nature to want to define/label etc, that's one reason why we like to know where our child is in relation to others in the class. We cling to 'ability' perhaps when we feel we have inadequate information about our child/the class etc.