First you have to make the case that it's actually a problem. Test scores didn't go up. So what? Is it actually a problem if the scores just stay the same? I mean, they can only get so high before you have to start screwing with the test to get different results. Why not just accept that the same scores year-over-year mean we're pretty much doing fine?

If we're going to have an enforced one-size fits all national curriculum, we can do better than the one we reverse engineer from some stupid standardized test.

Can we?

If we can't, then we have no hope of succeeding with a one-size-fits-all national curriculum anyways, so either way the point is moot.

Yes, boil the entire year down to one or two tests which have been shown to be a poor indicator of knowledge and performance. We'll just start holding back 25% of students. What could go wrong?

As is typical, easy solutions are often bad solutions. Teaching to the test would be the first-order effect, and that is a terrible use of class time.

Holding kids back a grade isn't really a thing they do anymore.

But, why?