In September 2020 in England, there was no regular at-home COVID testing (Lateral Flow), and secondary school pupils did not wear masks in classrooms.
With the March 2021 reopening, secondary school pupils did wear masks, and were asked to do regular lateral flow testing and stay home if they tested positive. Also we had started vaccination rollout, which has continued.
I believe that each of these measures to reduce transmission and protect against illness helps somewhat, and that all of them combined together help quite a lot.
So if most pupils continue wearing masks in class, and many pupils continue doing lateral flow tests, and more staff are vaccinated, and more parents are vaccinated, then there will be some increase in transmission due to schools being open, but much less than before, and even less increase in hospitalisations and deaths, due to the way vaccinations were prioritised.
COVID as a public health issue in the UK is definitely behind us, and resources must be provided to the NHS, to other frontline workers, and to people whose health or employment have been damaged by this pandemic. But, the situation is getting better and will probably continue to get better, and the worst is most likely behind us.