Apologies, I haven't read the full thread so it may have moved on since the first couple of pages but OP do you actually go to my school? Are our DC in the same class as I could have written your post. In fact many of us could have written your post!
Because, whilst it might not happen in every school this certainly happens in our school.
In DD's class particularly the same group are always chosen, without fail. In fact when pictures from their class are put on Twitter/Facebook it's like a game of bingo.
Now, whether it be a coincidence - they may just be in the right place at the right time. They might be a bit more confident, outgoing etc or it might be that it is because their parents are all on the pta, school helpers, school governors. That's not me being snippy either I'm just stating a fact as they are all lovely, lovely kids don't get me wrong.
I don't know why it happens? But I wish they could understand how it feels. To be trawling through the twitter/facebook feed whilst your anxious DC is away on a trip/residential, searching for a pic to reassure you, and you just see the same children pop up over and over again 
As I said it may not be the case in all schools of course, but it's certainly been the experience in mine.
(OP if you turn up at pick up wearing the Mumsnet scarf I know its you
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