Either being ill, schools having to shut or isolating as a contact?
I think we've been quite lucky. Both kids missed 2 days before half term as I was waiting for a test result (negative luckily), DD (in Y8) recently had a contact and was off for a bit but luckily the contact happened just before two days of planned school closure (nothing to do with COVID) so she only missed 7 days of school.
I have friends whose kids have been in and out loads and missed more school than they've had.
DD's secondary is small and split over two sites, so it was doing OK, but things really ramped up suddenly after half term and they had to move around curriculum because of teachers being off.
DS's primary seems to have not been affected much - a few kids being off waiting for test results (all negative).
I never believed in Sept that schools would make it to half term let alone Christmas without all shutting. I have kept back some days of holiday I can carry over as I fully expect some kind of shutdown or extended holiday Jan/Feb.