Scores of schools and colleges across the UK are closing early for Christmas, in defiance of No 10, while others are preparing for further disruption and possible closures next year as the Omicron variant sweeps the UK.

The Welsh government has announced that its schools will delay the start of the new term by two days “to ensure they have robust plans in place to move to remote learning if required”, while school leaders in England are preparing for an extended switch to online learning in January, with many ensuring that pupils take laptops and textbooks home for the holidays.

The Guardian has learned of primary and secondary schools in England and Wales that have decided to end term early because of staff shortages caused by illness and Covid-related isolation. While some schools have already closed on schedule, for most pupils the official end of term is on Friday, with schools in a few local authorities continuing until next Tuesday.

Parents at Mersey Drive community primary school in Bury were told by the school: “We have reached the point where we are unable to maintain safe staffing levels. After consultation with the local authority we have reluctantly had to make the decision to close early for Christmas in order to keep everyone safe.”

In some cases, schools and colleges have been advised to close or switch to online learning by local health officials, including Rochdale’s sixth form college and Falinge Park high school. Parents of students at the Rochdale college were told it needed to close because the “A-level timetable and the high population density” made it impossible to identify close contacts of a confirmed Omicron case.

The BBC reported that schools in at least 30 local authorities in England have moved teaching online during the last week of term. That has come despite a clear message from No 10 that schools should remain open.

Asked on Monday whether schools might close early for Christmas, a spokesperson for No 10 said: “Certainly we do not think anyone should be closing schools early, unless they have received advice from the local director of public health that it’s necessary on public health grounds.”

Stephen Morgan, shadow schools minister, said: “Children have been in and out of school facing ongoing disruption to education and wellbeing again this term. This cannot continue. The government has continuously failed to plan ahead, but must act now and use the Christmas holidays to prevent the chaos seen last January.”

Local authorities in England, Wales and Scotland are making preparations in case of school closures next month, or delaying reopening to allow for testing.

A spokesperson for Southampton city council said some schools there were “postponing their return date by one to two days to ensure a thorough testing programme is in place to enable them to welcome students back safely … The situation is under constant review, and we are on stand-by to support schools in implementing any new guidance.”

Other areas are preparing to take further measures such as reintroducing “bubbles” of students to minimise the spread of infection, and reintroducing social distancing measures such as staggered starts and restricted use of school facilities.

Jeremy Miles, Wales’s minister for education, said all schools should plan for measures based on the government’s “very high risk level” of infection control, and that he was issuing legal authority for schools to operate staggered start and finish times.

“I am providing all schools with two planning days at the start of the spring term. This will allow time for schools to assess staffing capacity and put the necessary measures in place to support the return of all learners,” Miles said.

Schools in Guernsey are closing early, the island’s government has announced, before the scheduled end of term on Wednesday.