How do you expect anyone on this board to be able to answer that?
Taken from the inked article:
"Sage looked at the modelling for nine different scenarios outlined by the Department for Education, from total closure to full reopening."
So, you'd have to contact the Dept. for Education, but obviously they must have come up with 7 potential scenarios somewhere in between the two extremes. Maybe Sage and / or the government came up with several others of their own, and finally settled on this particular one, - which doesn't seem to have particularly widespread support.