At the moment, case numbers are in the high 30,000s. At the end of November, there were less than 15,000 cases a day. Cases are not going to half by the end of the week...
If we reopened schools in just over a week's time, then we'd very quickly end up in the situation we were in at the end of last term- i.e. cases sky rocketing, huge numbers of students self isolating, schools having to close due to having too many staff off, etc. etc.
When we reopen schools, it needs to be done sensibly and carefully, not just as a free for all, and that will take proper planning, proper bubbles, probably some kind of rota system and an enforcement of social distancing in schools. Masks alone are not enough, and some kids will just refuse to wear them.
Schools have bubbles bursting now, never mind if we reopened as normal.
Also, "the vulnerable" make up quite a large proportion of the population, and vulnerable school children cannot have the vaccine. To vaccinate all vulnerable staff would take more than a week, and then it would take another week for immunity to kick in. Plus it's those with vulnerable family members, and children with vulnerable family members, too.
If we reopen schools too soon, it will prolong lockdown, prolong the economic damage to the country, will probably lead to schools closing again on an adhoc basis (so key workers will have no childcare)...
It's far better to wait, plan and do things safely.