I think schools should be able to do what works for them within set safety guidelines (eg a maximum number of children, years or days or a set amount of distance between desks or something).
Year 7 and 8 would be our absolute priority at the school I'm in (they're our oldest two year groups with senior school exams and lots of practical assessments). But those two year groups are probably at the absolute bottom of the pile of government priorities.
In the summer, when Year 6 were prioritised, we had no choice but to bring Year 6 back (a random, middling year in our school) and leave our year 8 leavers at home with no exams, no transition and very little in the way of leavers' events.
I think it will be the youngest ones but I don't agree that they are in most need. They have the most time to catch up educationally and, although they are losing out socially, I don't think it has as bad an effect on a 5 year old as it does on an 8, 10, 12 or 16 year old.
It's a tough call when they all need to be in school though.