The problem is that from September to December, rates in secondary school children went up 50 fold, way above increases in other parts of the community.
When secondary school children went back in March, the increase in covid rates was relatively low, lower even than primary school children, who are not so susceptible.
So it would appear that the increase in mitigations, with masks and testing, had a significant effect.
Presumably this is part of the reason that scientific opinion was in favour of masks being retained.
I'd like schools to be as free of covid as possible and to be able to stay open effectively, myself.