What we need to know is how many children test positive during the 14 day quarantine period, and I believe that families are under no obligation to tell the school if a child becomes ill at home. They can, and they should, but there is no legal obligation to do so. So realistically, schools don't know how many pupils the virus is affecting. If we had a good track and trace system, we would be able to know and schools could adjust their measures accordingly, but they don't know. It's a bit crap.
This and it's more than a bit crap, it's actually putting everyone at risk. In Germany they test close contacts of positive cases, which is probably why they're keeping the virus under much better control than us.
In the Israeli school outbreaks, 60% of children were asymptomatic, so there needs to be - at a minimum - some studies of the cohorts sent home when there's a case in school to try and quantify how much transmission is happening.
In theory at the moment, you could have children sent home for 14 days due to one or two classmates testing positive, they could then be asymptomatic or symptomatic without any of the 3 testing symptoms and infectious during that period (and would test positive if tested, which PHE actually says you should never do - only test with 3 symptoms) and then pass the virus on to a parent or sibling in the home, who then goes on to become infectious AFTER the 14 day period. Since current ideas are that the presymptomatic period could be important, any parent who then goes out to work / the shops could be then spreading the disease within the community. This is how schools will drive spread.