2 things to factor in: DofE has sent out a missive to secondary schools reminding them that if they are due to host exams in January, they are expected to have plans in place to still do that, if that means having additional exam invigilators trained and/or other staff trained to carry out exam officers work, they are expected to have made said arrangements- which doesn't sound like a plan to shut all secondaries beyond the first work.
Then also the plan to test in secondary schools to pick up the asymptomatic cases.
This makes sense to have all secondary kids back in 2nd week to test them all in the most efficient way. Any asymptomatic cases will shut that year group down, many schools will close after thats done anyway.
Expect the school testing programme to move out to the primaries shortly afterwards, although that will be harder to do given the age of the kids involved. Lots of asymptomatic cases will close schools for a fortnight anyway.
The mass testing of kids in secondary schools needs schools open to do it.
I'm in tier 4, when we go back, we will have just have over a 2 week school break when it's pissed it down, nothing is open, you weren't allowed to see anyone on Christmas day, if this doesn't get the numbers down, not much else will.