With regard to the church cluster - maybe their children walked home alone but I'd imagine some were driven home by adults/ picked up from the church by their parents. It's not as simple as saying the child infected their parents, although they may have done.
Supermarket floor workers are often late teens or early twenties and like all young people think nothing will ever harm them. They are also likely to be affected by peer pressure. Younger people are lower risk but they are not immune, a few do die and they have a right to be protected, even if it from their own lack of thought. Their managers are likely to be older and need more protection.
We clapped for all the key workers and we sterilised bin handles/ door bells/ letterbox flaps. Not as vigorous with that now because most surfaces are not a high risk (metal and plastic worse than soft things), and in this weather viable virus is unlikely to be present outdoors after a very short time.
If schools made the 6th form wear masks - and they should, at least in high incidence areas - it would be seen as "cool" and younger children would probably copy.
There are 2 bits of evidence that would make me support masks in secondary school - graphs suggest that high incidence areas control outbreaks faster when they test children and that masks are actually working in reducing infection. Neither statement is very soundly based but we should operate on less than perfect evidence with a highly infectious virus.