I agree that dividing up by group is stigmatising for children- my bottom sets are very aware of being bottom set, and it is damaging for their esteem. Equally, they wouldn't like being forced into school when no-one else is. This would all impact the amount of productive learning being done.
I wonder if the answer is to have Y10 and 12 go back in secondary, and the youngest children go back in primary? With older primary school children, it's at least a bit easier to work at home, and secondary children are less likely to need childcare, and Y10/12 will have the least time to catch up.
A lot of Y12 classes are small enough to socially distance at least, and Y10 could be split up between more teachers- maybe half the time with a subject specialist and half the time with someone else? You could redo the timetable, to minimise who was in on each day, maybe?
In primary again, classes could split between more teachers and rotate.
Rotating who is in each day doesn't really help people get back to work, and in secondary I think you'd end up with each year group missing out on some subjects. One day a week might help primary students educationally, though. One week on and one week off might work better for secondary.
I agree with whoever said that you will still get issues with schools not being able to open at all at times if e.g. a cough goes round the staff. Unless they decide that we're not to self isolate- which puts people at risk.
I think however it is done, it will make some people really unhappy.
I almost think it would be better to put everyone back in for a few weeks- catch up what we can educationally, and then accept we will need to have a second lockdown quite quickly.
All the suggestions about opening schools over the summer etc. assume we won't need a second lockdown once schools reopen- but we definitely will, it's just a question of timing!