My daughters primary school hasn't had more than 8 key worker children in at any one time since lockdown began. Four of those children come from 2 families.
When fully open there are 210 pupils. If most people have 2 kids in the school then that's 105 families as opposed to 6. Then factor in having all staff (20) in instead of 4 (caretaker, office staff, one teacher and one TA.)
My eldest is at secondary which is 7x the size of our primary.
My kids want to go back to school. I will send them in when they are able to go back. But I can see that opportunities to be exposed to CV19 will increase.