Account,
I'm sorry, I don't seem to have explained myself very well.
The 1 in 1500 is the community cases - it's the large scale randomised testing outside hospitals and care homes.
The risk is to adults in school, vulnerable children in school, and adults in the community, and it is the magnitude of the risk to them that i am trying to quantify (a poster on another thread described it as 'tiny' so I was a little disconcerted to find it is c. 1 in 100 )
On Day 1, the school has 1 case in 1500. That child spends either 5-6 hours with a single group, or 5 -6 separate hours with different groups of 30, without social distancing and without a mask, in a poorly ventilated room.
The number of cases in the school and in the community by day 14 could be really quite high ... though case 1, if asymptomatic, may never know.
From everything we know about the virus, transmission in schools seems inevitable - it spreads in crowded, indoor places with lots of people and poor ventilation, and that pretty much describes your average secondary....