> The issue is that all teachers significantly
> over-predicted their students grades,
> but less so at high-performing schools.
There are teachers in comprehensive schools that are tough-markers, too. My wife, who is a teacher in a London school, was annoyed in both directions - there were teachers that she felt had given away high grades, but there was also a teacher that had given the kids in her class grades lower than she expected they'd get (as her identity is tied to being overly careful and she liked to grade kids harder in mocks, etc).
What she felt was bad was that cohorts of kids were often very different year upon year. Last year there were particularly bright kids doing Physics in her school, but this year there was only a few kids and they were much worse. But the algorithm awarded them grades above what she expected.
There were also kids that she expects would have worked hard before their exams and gotten results above what they were given. They didn't get given the chance to do this.