Education Secretary Nadhim Zahawi is making contingency plans for rising rates of staff absence in England's schools.
One in 12 teachers was off during the first week of term.
Schools have reopened across the UK with a variety of Covid restrictions in place.
What's happening this term?
Across the UK, schools are expected to offer face-to-face teaching to all age groups, and provide effective online learning for pupils who are absent or isolating.
The specific Covid measures in place vary across the nations.
In England, secondary school pupils now have to wear masks in class as well as in communal areas. The guidance will be reviewed on 26 January.
A review of the use of face masks in schools to stop Covid spreading said the evidence is "not conclusive", but suggests they may help.
The government is also making 7,000 air cleaning units available to early years settings, schools and colleges - although Labour says this is not enough.
In Wales, schools are getting an extra two days to make safety preparations. All Welsh schools are expected to be open by 10 January, with staggered start and finish times.
All school staff and secondary school pupils are asked to take two lateral flow tests (LFTs) a week (three in Wales).
In Northern Ireland, schools are reopening with the same measures in place as last term.
In Scotland, social distancing remains in place, pupils are separated into groups where possible, and teachers are required to wear masks when in close contact with pupils.
Can a child go to school if someone in their household has Covid?
Yes - as long as they don't have any of the main Covid symptoms, under-18s don't have to self-isolate if someone else in their household has Covid.
However, If they are over five years old, they are strong advised to take LFTs before leaving the house for at least seven days.
Similarly, whole classes are no longer automatically sent home if a pupil or teacher tests positive.
What are the risks of schools staying open?
There are two main risk factors:
Towards the end of last term, staff shortages meant some schools sent classes and year groups home.
In several parts of Wales, remote learning completely replaced face-to-face teaching.
Supply teachers are in short supply because of the high levels of absence. Many are already working in tutoring organisations as part of education recovery plans.
England's Education Secretary Nadhim Zahawi urged retired teachers to return and join teacher supply agencies to fill gaps, but they are unlikely to be in place quickly.
He also suggested schools could merge classes in the event of teacher shortages.
But teaching unions said there were logistical difficulties and safety issues with teaching large groups of children.
What happens if there is an outbreak?
Under England's guidelines, a school must get in touch with public health officials if five pupils, teachers or staff in close contact test positive for Covid in a 10-day period.
School management must then work with local health officials to mitigate the spread of disease.
Could schools close again?
In England, short-term attendance restrictions - such as sending home a class or entire year group - can now be used only in extreme cases, "where all other risk mitigations" have failed, and under public health advice.
Before Christmas, this test was met in at least 30 local authority areas and classes were sent home.
Schools must be able to deliver remote-learning provision in the case of absence or closures.
Many local authorities told the BBC at the end of December that online learning was ready in case schools could not open fully in the new year.
Some schools sent pupils home with laptops and learning packs in case of emergency closures.
While vaccinations are not a 100% guarantee against infection, they should give extra protection from disease, which is generally mild in young people.
All children aged 12 and over are being offered two doses of Pfizer, with the second jab typically given 12 weeks after the first.
Children who aren't considered to be at high risk from Covid should wait 12 weeks after a positive Covid test before being vaccinated.
The Joint Committee on Vaccination and Immunisation (JCVI) has also recommended that booster doses should be offered to children aged:
A low-dose version of the Pfizer vaccine has also been approved for five to 11-year-olds who have health conditions that put them at greater risk from catching Covid.
The JCVI also recommended that primary school children who live with clinically vulnerable adults should also be offered a jab, but it's not yet clear when children might get the doses.
What about ventilation?
Ventilation is one of the key measures designed to keep schools safe from Covid.
Some 300,000 carbon dioxide (CO2) monitors had already been promised for schools in England to help identify areas with poor ventilation. But teaching unions complained this would not solve the problem.
The Scottish government gave £100m to local councils to help fund Covid improvements, including hygiene measures and ventilation.
The Welsh government has a £6m programme to increase air circulation and purity which will provide 30,000 CO2 sensors and 1,800 ozone disinfecting machines.