Being as cases have still dropped seems to work well.
Initial infections in schools, by and large, mirror cases in the community - it's a different case once a number of infected individuals are present in the same school (see outbreaks of 80 and 100, and 30 including new variants, in schools and colleges recently).
So cases in the community, as Boris has said, have dropped due to lockdown. So have cases in schools. They have dropped more in secondary schools (masks, testing) than they have in primary (no masks, no tests).
Other mitigations - ventilation, handwashing, class-sized bubbles in primary - have had some effect, no doubt, but since they were also present from Sept - December, when cases skyrocketed, they clearly aren't sufficient on their own. Interestingly, there was a recent paper from the US that said the increase in infections in families containing school age children only reduced when at least 7 mitigations were in place.
It is yet to be seen how high the community cases will rise as lockdown eases, but to return to only the mitigations present from Sept - December seems foolish, because they are proven to be ineffective in preventing a significant rise of in-school infections in the face of rising community cases.