We moved DS1 to a private school in September at the start of Y3. DS2 went at the same time, for the start of Y1. This was so we did not have two school runs in different directions, different holidays, and also to make it easier for DS1 to settle. He had lots of friends at the old school and has missed them - I think it would be hard if DS2 was seeing the old friends daily and DS2 was not. DS2 loves dressing up so he is very pleased with the amazing range of uniform required for a prep school and would have hated to miss out.
DS2 has taken to the new school like a duck to water. He is in a much smaller class (12 not 25) so is made to read every day, whether he likes it or not, and can do extra maths if he wants to (he does). He had no particularly special friends at his other school (apart from 2 who also left at the end of reception) and was a bit lost in a two form entry year.
DS1 has settled in now, and is making friends. His class is 23 not 31. He's found it harder to adjust to the workload, and the lack of close friends. We'll do playdates with the new school friends over the holidays. Until now we've been catching up with old friends. I'm pleased that the school's expectations of the boys are higher now - DS1's reading age is his actual age, but they would like it to be a bit better than that, and I think he can manage with support, which he is getting.
It's the secondary schools here which are my concern, and so I just said to the other parents at the old school we were going private sooner rather than later. It helped that other people were doing the same at the end of year 2, and there were lots of people moving out of London at the end of year 2 as well.
There are some amazingly well off parents at the new school.... But my boys felt deprived before, because the kids from the council estate all had Wiis and X-Boxs (apparently) and we did not. Still don't!. There are differences at all school, and like others have said you need to try not to get wound up by them.