Would very slightly reducing the rate of case growth be particularly advantageous?
I can't see any advantage in slowing spread in under 12's - the vaccine is way too far away, the slower it spreads all that happens is more repeated bouts of isolation in the groups and the higher the risk to teachers etc. (as they'd be in repeated contact with infectious students)
12-16's there may be a vaccine soon enough that it would be worth delaying, but I'm not so sure, it's still a long time away and current restrictions are not effective enough to reduce spread so you have the same as the younger groups really.
In Adults, I guess it depends if we think there are a lot of people who are reasonably vulnerable and would get vaccinated now (and then 8 weeks for protection) such that it's worth minimising spread until then.
The 6-15% of masks, or even 25% is I don't think particularly worth it though, I simply don't believe that much additional spread would happen in environments with masks. A year ago we had supermarkets without masks and R below 1, so I think supermarkets simply aren't much of a driver, so a 25% reduction in spread there will be irrelevant compared to a restaurant, a pub, or a dinner party.