In my DDs secondary school, there have been no cases that we know of. The school haven't told anybody to isolate.
The only person that I actually know, that has tested positive is a nurse, who was told she had been in contact with a positive case (patient). The school has been back since September with no cases. They update us every week and no cases yet.
The cases started rising in August when lockdown was stopped, they have obviously just risen as everyone has gone back to work and eating out, getting haircuts etc, not just because DC's have gone back to school.
I think Boris's ultimate plan is to lockdown for a month with just some people going to work and DC going to school. Then in December cases will have dropped, maybe Uni cases will have burned themselves out (they are already dropping in my area).
Then Boris will say "look it worked, cases have dropped even though DC are at school."
I do wholeheartedly agree with DC going to school, they can't miss any more education, especially the exam years.