It's hardly lazy. It's cautious. Children don't enter the classroom in their exact order 2m apart. They share toilets, touch the same door handles and often share buses.
Exactly.
DD's school has had 3 year groups isolate, each 1x, in no further outbreaks or cases in those year groups after the isolations - in other words, outbreaks nipped in the bud by timely complete isolation of the year group.
I would MUCH rather that then a constant series of 20 kids here, 40 there, then another 15 including 5 of the original 20, then another 18...
It also follows the science of an airborne virus - anyone spending an hour's lesson with a positive case is at risk of infection, especially once airflow is taken into account - see the very useful modelling of early outbreaks in a restaurant, office and bus.
Just because the DfE helpline is under political instruction not to send home larger numbers doesn't mean that the isolation of the larger number isn't a MUCH better plan to actually get infection under control.... in fact, when public health manned the helpline (early in September), large groups WERE going home. It was only when it swapped to untrained call handlers run by the DfE that the advice changed....