Face coverings remain compulsory in Scottish schools, while students and staff in some parts of England are also being asked to wear masks again after a rise in coronavirus cases.
Mask-wearing stopped being compulsory in May, but the government is reserving the right to re- introduce it under its winter plan.
What are the rules about masks in schools?
Face coverings are no longer compulsory in schools in England or Wales, although they are recommended in crowded spaces like school buses.
However head teachers and health officials can ask staff and pupils to wear masks on school premises in response to local circumstances.
Education Secretary Nadhim Zahawi confirmed that mask-wearing in all English schools could be made compulsory again under the government's winter contingency "Plan B".
In Scotland, face coverings remain compulsory for secondary school staff and pupils on school premises after after the Scottish government decided against lifting the measure. Primary school staff only need to wear masks when moving around the school or in communal areas such as canteens and staff rooms.
In Northern Ireland, they are required in class for at least the first six weeks of term.
How many pupils are missing school because of Covid?
Department for Education (DfE) statistics show that the number of pupils missing school in England due to Covid-19 has risen over the last fortnight. On Thursday 14 October, 2.6% of all pupils - about 209,000 children - were not attending class for coronavirus-related reasons.
In the snapshot two weeks earlier on 30 September, the figure was 2.5%, or about 204,000.
The main reason for absence was a positive test for coronavirus, affecting 111,000 pupils compared to 102,000 a fortnight earlier.
In Wales, there have been more than 10,000 cases of Covid among pupils and staff since the start of term.
However, preliminary absence figures for the last week of September were down 44% on the previous seven days.
In Scotland, the latest figures show 13,026 pupils (2.3%) weren't in school on 8 October for Covid-related reasons. The highest absence rates due to Covid were recorded on 7 and 8 September, when 5.7% of pupils were out.
In Northern Ireland, 3.1% of pupils - about 10,000 - were learning from home due to self-isolation or social distancing during the second week of the autumn term.
That was the highest proportion since all pupils returned to face-to-face teaching at Easter.
What happens if a pupil tests positive?
They must isolate at home for 10 days but their classmates, siblings and parents don't need to quarantine unless they also test positive.
The rules changed to prevent situations where whole classes were sent home because of one case.
What's happening about vaccination?
All healthy 12 to 15-year-olds are being offered a single dose of the Pfizer vaccine to help limit school disruption.
Evidence suggests one dose cuts the risk of catching the Delta variant by about 55%. It also reduces the chances of people getting very sick or spreading it to someone else.
The rollout has already begun in England, Scotland and Wales, whileteen vaccinations are expected to start in Northern Ireland in November.
The UK's chief medical offers said a second dose shouldn't be considered before Spring.
What other measures to prevent Covid are in place?
Social distancing measures have been relaxed for schools in England, Wales and Northern Ireland.
But as with face coverings, head teachers can decide whether any restrictions are still needed. They can choose to keep one-way systems and crowd-control measures in place.
Government guidance says they can also temporarily reintroduce class bubbles if they wish.
In Scotland, more restrictions are being maintained.
On school premises, all staff must stay at least 1m (3ft) from pupils and colleagues.
Across the UK, all school staff and secondary school pupils are asked to do regular lateral flow tests.
What about ventilation?
The Department for Education pledged to provide 300,000 carbon dioxide (CO2) monitors to schools in England from September - to help identify where airflow is limited and viruses may more easily spread.
The Scottish government has given £90m to local authorities £90m to help fund improvements to schools to help tackle Covid, including enhanced hygiene measures and ventilation. It provided a further £10m for ventilation in August to ensure CO2 is monitored in all schools and early learning settings.
The Welsh government has a £6m programme to increase air circulation and purity. Some 30,000 CO2 sensors and 1,800 ozone disinfecting machines will be made available.