Hi PickledMoomin:
I agree with SoupDragon - please try (and I know it is very hard sometimes) to make it clear to your DD that it doesn't matter what group she is in, as long as she's trying her best.
The reality is there are 30 kids in her class (or thereabouts) and top table can only accommodate so many people. My DD2 has often been moved down groups and has been absolutely devastated - she's hyper aware of 'top table' because one of her great pals is always top table, so she wants to be there too.
I think you have no choice really but to accept the teachers decision here. It could be for any number of reasons, including the possibility that your DD doesn't pay attention if sitting on a table with a friend (which has been the case for my DD2).
In Y1 for DD2 she was moved down to 2nd reading group and we had to weather 4 months of repeated reading books - the low point being 3 weeks of Big Panda, Little Panda (I don't approve of book burning - but genuinely hope this is first on a bonfire!). Some very calm Dad here on MN made a radical suggestion - you don't have to do what the school says.
Don't like guided reading books: read them 1/2 times a week, and then read what you like
Don't like spelling list: get spelling work books and do more at home/ do on-line games/ etc...
Don't like maths homework: join on-line tutorials/ get worksheets off the web/ get workbooks.
You don't just have to do what the school sets.
It's very liberating and means that your own internal sense of what should be done when (possibly in consultation with draft national curriculum statements/ teachers/ friends/ family) can be satisfied.
Finally I will say this. DD2 has frequently worked hard to get back up to top table - and that is beneficial.
HTH