Schools have 7 year groups 

Only Reception, year 1 and year 6 will return 

so that's 3/7th

if class sizes are 1/2

surely 1/2 > 3/7

so every child will be in a classroom no?

moreover https://www.theguardian.com/world/2020/may/18/french-minister-tells-of-risks-of-missing-school-as-more-pupils-return-covid-19