The thing is, after the first cakll to the DfE helpline 9when they send out the standard letter for you to send out), there is NO national plan at all.
The helpline doesn't want to know if it isn't your first case - they advise on the first case and never again.
After that, it is the job of your local public health service, which have very different policies by area. In general, those historically in ier 1 areas seem to be quite risk averse, those in ex Tier 3 areas jave so many cases they advise cklose contacts only.
It also depends by school. Many started by sending whole bubbles home, either 'just in case' or because they did not have good systems for identifying close contacts. They then realised that the school would close very rapidly, and that parents might object, and in line with the mood music [school blame] from the Government, have now gone for close contacts only.
There is a strong presumption in all guidance given that teachers have not been within 2m of anyone (even when it is blatantly untrue) in order to keep schools open that should probably be shut through lack of staff.
Note that NONE of this is the fault of schools. They are having to respond to guidance that is driven as much by PR and political desire as by any form of health concern.