OK I spoke to her this morning
I said "good morning" and smiled, then said "I hear ds1 took a science test yesterday?" and she gave a sort of embarrassed grin and said "Ah, yes, erm, I'm not really sure quite how to tell you, you see there was this year 6 SAT paper I happened to glance at, and I saw some of the questions and thought Greencuff would be able to do that..."
to which I smiled and said "Yes, I had a couple of friends confirm that it was the Year 6 paper after ds1 told me about some of the questions" (she looked a bit shocked by that, pmsl )
I then asked in a very non-confrontational way what the purpose of it was - and she said they are considering moving him up to KS2 for some science lessons - apparently at first perusal he easily passed the test yesterday even though there were some inevitable gaps in his knowledge because of his age, eg he didn't know what a 'variable' was. Also he apparently didn't do the ice cubes/stopwatch question because he wanted her to bring him a real stopwatch/ice/bags and DO the experiment, he said "how am I supposed to use this equipment when it's just drawn on the paper"
I expressed (as I always do) the importance to me of keeping him socially in his peer group and continuing to work on integrating him rather than singling him out, as I don't want him to spend him adult life walled up alive in some vault in Oxford like a mad monk
she agreed and said that he will continue to spend as much time as possible doing choosing/play/normal stuff with his peers.
He is going to be sitting the other science paper today and the head of KS2 science is going to mark them, then they are going to decide how best to proceed.
I still think the way this was done was uncharacteristically cack-handed for this teacher, but having spoken to her I can see why she decided to do it. I am still not very happy about him sitting a Y6 test on his own - but she has assured me that he loved it, was fired up and on form, and wanted to do the other one. So I am going to let things unfold [grudging]