My son is also finishing year 1, going into year 2 in September. The school where he goes is not a "primary" school, it is an infants with a juniors over the road where he will automatically go after finishing Infants. They always used to split the classes before joining Camden but this year they have split the class at the end of year 1, rather than the end of year 2.
Saying that, he is going into a year 2 class with 2 of his 4 best friends and he is also a very sociable boy who has no trouble making new friends - I am not bragging, I know how different children can be - my eldest girl is a bit shy, finds it hard to make new friends, etc. whereas my son makes friends very easily.
My son started playgroup and make a "special" friend. When they started the nursery together attached to the infants' school, my son got afternoons and his friend got mornings. Because I didn't realise how sociable my son was at the time and I wanted him to settle in as easily as possible, I asked the nursery if he could change from afternoons to morning so he could be with his friend. They allowed this. The nursery always thought my son were more dependent on his friend than the other way round, but in fact the reality was the reverse, which they found out when my son and his friend took a holiday at differen times - my son just made new friends and still enjoyed himself whereas his friend had a difficult time as he was so dependent on my son.
When they started reception (last Easter), the school split up D (my son) and O (his friend) as they felt O was a "bad" influence on my son. My son settled in easily to his new class but O has had a few problems.
When they started year 1, O's problems escalated (his mum is going for funding/statement, etc. at the moment) and he was moved to another year 1 class as the teacher could not cope with him disrupting the class. The new year 1 class could not cope with him either so he was put in a year 2 class although he was only 5 at the time. After Easter his school day was shortened to finish at 11:30 (only 2.5 hours in school) as they felt they did not have the resources to keep him in school for the whole day. When his mum queried this - saying what on earth would he possibly learn in only 2.5 hours a day at school - they said he wasn't learning anything anyway.
It is only now that he is back up to finishing at 1:30 (2 hours early) and they were hoping that they could have him back full-time before the end of term but it doesn't seem this will now happen because his problems have not been "sorted" and he is still disruptive, threatening, etc.
When the year 2 children sat their SATS, he was moved back into his original year 1 class and he loves this teacher now, having spent time with 2 other teachers, although he is still sometimes disruptive, rude, etc.
Because of O's problems, O's mum was a bit concerned about how he would settle back into school after the summer holidays as it always takes all children a few days to settle back down after such a long summer holiday but with O's problems she felt it would be even worse and she didn't want him starting year 2 on a short day again.
Because he likes his teacher so much, next year she will be taking a year 2 class and O will be with her as he has got a rapport with his teacher and she does seem to sympathise with him.
I seem to have waffled and gone off the trail a bit - which I seem to do quite often which is why I don't always post a message!!! - but I would say speak to the teacher and I can't see why they would not be willing to accommodate moving one of your daughter's friends (or your daughter) into a class with at least one of her special friends, unless of course they have a valid reason - sometimes some children do not mix best with other children, i.e. if you have 2 disruptive children in a class, they will split them as they will not be encouraging each other to disrupt the class.