As a child I was in composite classes - a 4/5 (in Y4) and a 5/6 (in Y6). I am pro them, despite the 4/5 year being the worst of my primary life (due to the teacher). You learn more socially, the academics aren't that much of an issue as a decent teacher will plan on how to stretch the more able anyway so have extra, harder activities. There's a huge range of abilities in that age group anyway, so a bright Y5 will be brighter than a slower Y6. A slower Y6 won't be the bottom of the class all year, even if they are bottom of the Y6 group. If you're top of the older class, you would be no matter who you're with... so does it matter who you're with?