Presumably teachers are off sick at times and the whole class doesn't collapse and fail to achieve?
yes but depending on the type and length of sickness, the teacher may either set the same work that they would have done anyway, to be covered by the TA (primary).
Or the head of year/dept will set the appropriate "next stage" lesson, to be implemented by a cover teacher. Don't forget teachers are required to plan in advance, they dont make it up day by day, so colleagues will have a very clear idea of where in a scheme of work the pupils are up to, and what comes next.
Or in the case of long term illness they will get long term appropriate supply teacher in.
The teacher being absent is very different to a child being absent. The school have a duty to catch up missed work/provide the appropriate work if a teacher is absent - not the case when parents are keeping kids off willy nilly.
It amazes me the number of parents who are adamant that a week off does no harm, when a child may miss a whole subject in that time eg "there/their/they're rule" or how to tell the time, or a whole times table, or rules for plurals, or long division etc. In a week they may miss several of these topics, which may not be revisited till the next school year. Who is going to help that child catch up? In my experience as a teacher, not the parents!