Right have spoken to the school, and I think my feelings are now very clear, remained polite but I was not happy.
I played ignorant to begin with - I said have the groups for the trip been organised? And I was told yes and when asked was given the names of the other children in the other group.
The other group is my ds and the other boy, and all the girls of the other class. (My ds although has no issue with girls (older sister) doesn't play with girls at all in the playground).
I asked why is he with the other class, and was told that he'd keep the girls in check as they all look up to him. I said he's 5 how's he keep girls in check, and was told due to his size (he towers over his classmates) they adore him, and will follow him, which will make it easier.
The other boy's mother was not happy about him going on the trip (getting lost) and therefore she wanted my son with her son, as her son would remain with my son.
So I said let me get this right, my son is looking after these kids, has he done something wrong. Oh no he's the most sensible and most behaved one, he sets the best example.
I explained that this was getting ridiculous, he was being punished for being good. I said you are separating him from his friends on the trip not because he will misbehave but because he is well behaved, I said you separate him at lunch time from his friends not because he's a fussy eater, but because he behaves and eats well, and you want him to set an example. I said you are putting too much pressure on him, and I want him to be rewarded for his good behaviour.
And the teacher responds, well he said he didn't mind when we asked him. No of course he said he wouldn't mind, again because he behaves and doesn't kick off when he has to do something that he doesn't want to do, you have taken advantage of it.
Anyway the long and the short of it is, I agreed to the other boy going in the group with the girls, and 2 of the girls are going with the other girls and 2 of the boys from my son's class are going with him. He will be moved back to the regular table for lunch times.
And it is unlikely there will be any class mixing up at the end of the academic year, although they wouldn't commit to that yet.
I am very glad I spoke to them, but how utterly ridiculous.