Does it imply that school children have previously had delta but young adults haven't? Why would that be ?
I think the hypothesis is that school children mix in smaller groups than pub/restaurant/nightclub goers, and that fewer of them managed to avoid infection via vaccine than the young adults. So the "pub wave" that we saw in July/August that the young adults were in, was mediated by vaccines as well as previous infection. That has left a larger susceptible pool in the age group now that vaccine alone is less effective. So the 10% maybe who avoided it due to too many of their friends getting it but not being unlucky themselves and the 30% who avoided it because of vaccines was enough with the mitigations to keep R around and even below 1 even as vaccines waned. However with the lower effectiveness of vaccines you might now have 40% in the age group susceptible to omicron which is enough to drive the rise.
In the unvaccinated kids, it got to the over 70% in every school class because there were no effective mitigations, so now they're more protected because the overall immunity from infection is higher.
This would also explain why the omicron wave starts dropping relatively quickly if it does (it appears to in Gauteng) - the susceptible population is simply not as large as feared.
The main problem if this hypothesis is true of course is that the older and more vulnerable age groups will have less infection or infection+vaccine immunity so it will spread faster in those groups. The only news if it is true, is that fewer mitigations would be required to limit the spread in those older age groups because there will be less infected younger ones around.