The return to secondary school in January will be staggered in England, with some starting online rather than in class, the BBC has learned.

It will allow headteachers to set up a Covid testing scheme announced this week - but with exam-year pupils going back to start term as usual.

The National Education Union said making the announcement right at the end of the school term showed "panic".

School leaders will be expected to run and manage the testing regime.

Infection rates

Secondary-school-age children have among the highest infection rates, but an official study suggests virus rates in schools reflect the levels in their local communities.

Virus rates have been growing fast in some areas, including London and south-east England, in recent weeks, with many schools affected.

The analysis of tests on 10,000 staff and pupils from Public Health England, Office for National Statistics and the London School of Hygiene and Tropical Medicine found 1.24% of pupils and 1.29% of staff tested positive for coronavirus in schools.

However, the impact of those cases will have been felt by many more, as close contacts were required to go home and self-isolate.

The government is insisting the change to the start of term is not an extension to the school holidays and primary schools will not be affected by the move.

'Army of volunteers'

But the move comes after the Department for Education instructed all local authorities to keep schools open in the final days of term, despite several initially telling parents that schools would close early and head teachers calling for more flexibility for online study.

Education Secretary Gavin Williamson is due to give more details later on Thursday.

But teaching unions are already angry that they will be required to train and deploy an "army of volunteers" to run the testing.

Joint general secretary of the NEU, Mary Bousted, said: "Armed with a 30-minute training video they are being asked to administer tests to adolescents - who may have their own views about what is quite an invasive procedure."

Dr Bousted added that contacting parents for permission to test was a huge task for schools, let alone running their own mini test, track and trace systems.

"Yet again ministers fail to understand the fundamental issues involved, and the effort and time it takes to operate Covid security procedures in schools."

'Chaotic approach'

Patrick Roach, general secretary of the NASUWT teachers' union said: "Yet again the government is announcing significant changes affecting schools with little or no time to prepare before the Christmas closure period."

He said it was not the responsibility of teachers or school leaders to undertake testing of pupils or employees.

"The government has to ensure that it puts into place all the necessary resources needed to deliver the practical and financial support to schools to ensure safety in schools," said Dr Roach.

Association of School and College Leaders general secretary Geoff Barton said: "We are very concerned about the feasibility of setting up a testing programme at the scale envisaged."

He added: "The profession is very willing to work with the government over how to roll-out mass testing, but ministers must understand that chaotic, last-minute announcements do not constitute a collaborative approach."