Note: I think the scaling might actually not be set so 80% get at least 100, but it's definitely a majority scoring at least that amount and an average well above 100. And honestly, the results that come out of this are pretty useless, for a few reasons.
There's not much scrutiny and oversight for the Key Stage 2 SATs, perhaps because the kids don't get any qualification from them, just a result, perhaps because they're the only tests kids do in Primary School - so about 4-5 tests (English and Maths, possibly also Science - not entirely sure), once a year. Compared to Secondary Schools which have literally hundreds of exams each year, often over several different exam seasons, plus several mocks done under formal conditions, and the exams lead to something so there's a lot more rules and regulations regarding their security.
Since those few tests are one of the main things a primary school gets judged on, they have heavy incentive to cheat. This can be done in a few ways - overlooking kids breaking normal exam conditions such as helping each other or quiet wispers, having posters/display boards with relevant/useful work still visible, or even more extreme things like teachers giving kids answers or hints. Obviously, not good when it happens. We have no formal evidence of it happening (I work in a Secondary School maths department) but there's a few primary schools we've noticed. The kids from there always seem to have considerably better results than their actual ability shows in year 7. On the old level system (levels 3-5), it could be a whole level different or more when we give them tests - to put that into context, kids are expected to make about 2/3rds of a level of progress each year, so this is like they've regressed 1-2 years of progress compared to what the SATs said they were at, right after they arrive.
Primary Schools also end up teaching to the test, since they want their results to be as good as possible. Well, okay, even Secondary Schools do that, but it's more of an issue with Primaries since they're feeding kids up the system. The kids aren't getting anything out of this, except maybe some ego from a good result. But their understanding suffers, since they're being taught how to solve SATs paper problems, not how to use maths in general. With GCSEs (year 11/16YO tests), you teach to the test and a kid gets a good grade. Their actual understanding may be a tiny bit worse off than if you taught more thoroughly and whatever, but their employers/colleges care more about that grade number for getting on the course.
On top of that, to get good results, what a lot of Primaries do in year 6 now is having longer Maths and English lessons throughout the first 2/3rds of year 6, then after the SATs in early May, they do no more Maths/English for the rest of the year. What that ends up meaning is that between May and September when they start Secondary School they do no Maths, and little direct English - and especially in Maths, their ability suffers as a result. We're getting kids into school who in theory are really good at maths, but in practice are struggling with questions aimed at middle ability kids, since they haven't done maths in so long. This bit does eventually even out a little, but they still end up a bit weaker thanks to it.
What my school has done in the past is to give kids a test just a week or two into year 7, and arranged ability based sets on the results of that test. The result would normally be pretty good - most kids, after doing tests at the end of the first term, would get results which more or less match their set - you'd normally see maybe 5-10% of kids moving up or down one set because we've noticed they're too adept/not strong enough at maths compared to the rest of the group. This year, we used the SATs results, for speed at the start of the year. And oh boy was it interesting. A good 30-40% of the year group had to move sets. About 2-3% of the year had to move by more than one set, indicating that they'd been massively misplaced. One of the middle ability sets had almost 60% of the kids move up or down, so it basically became a completely new set. You get the idea - these results were crazy inaccurate compared to our own measurements.
And the best part of all this? Secondary Schools get measured quite heavily at the moment based on kids making "expected progress". What is this expected progress based on, you ask? Why, their KS2 SATs results, of course! We get judged, for instance, on a kid making little progress from their fantastic KS2 result, when we know that result was probably gotten through cheating/coaching through exams and that we can't expect to match that, since the kid just isn't that adept at maths. Of course, that's not to say we're the only school facing that issue, but it's bad for us and it's also bad for balancing support - the school is expected to provide extra support to these "underachieving" kids, since well <i>obviously</i> they can do better if you look at their KS2 results, but of course this extra support has to come from somewhere, so it eats up resources that could better support other students who might benefit more from it.
Sorry this got a little ranty. Education in the UK is a bit of a mess at the moment, and the new KS2 SATs are certainly not helping the issue. But maybe this was an entertaining or at least enlightening read - or at least helped you get some sleep? Any of those would be a success I guess.
