I think that some of us (me included) imagine that classrooms are functioning as normal. Because I'm now retired, I'm dependent on what teachers tell me. Twice in the last week I've been really shocked how naive I've been.
First was a secondary school teacher saying that a third of her school's teachers are off either isolating or shielding. Consequently there's an unusually high number of supply teachers in who don't know the system or people to talk to (assuming those staff are in school), and so behaviour in school is sinking. Most teachers in school are losing planning frees to cover, and TAs are being asked to look after classes. Hence staff are very run-down which is also very bad for behaviour.
The other teacher told me 2 teachers had died in their school. (This has a devastating effect on pupils, who have probably have only ever known death in the context of old people.)
Two other facts. It's only in last couple of weeks that govt has accepted the need for adults and pupils to wear masks in the classroom. And England's Y11s and Y13s have been told their exams will be 3 weeks later to accommodate the disruption. With staff out and bubbles sent home, I would think most lost 3 weeks classroom time in one half term.