I've heard people say that before, that they can't be constantly in a composite class, but I figure it must be a myth. How would they get round the 1 class or 2 class schools in rural areas where the children would always be in a composite class?
tbh - i don't think it makes any difference - there are always a range of abilities and ages in a class anyway and the composite classes are limited to 26 (in Scotland anyway) so they will probably be in a smaller class rather than the 32 in my son's straight class.
DD1 goes to a school with 2 classes: YrR-2 and Yr3-6. It is a tiny rural school though. My primary school as a child had combined classes despite not being a particularly small school so that in one year I was in the oldest group in the class and the following year, in the youngest group. Don't recall any problems.