Video report by ITV News Political Correspondent Chloe Keedy
All pupils in exam years will return to secondary schools in England from January 11 while the rest of secondary and college students will go back full-time on January 18 - a delay to the planned return to school.
The majority of primary schools in England will return after the Christmas break as planned on January 4, but secondary school pupils will return later in the month.
Primary schools in a “small number of areas” of England where Covid-19 infection rates are the highest will not reopen for face-to-face teaching to all pupils as planned next week, he told MPs.
The reopening of secondary schools is delayed with years 11 and 13 returning on January 11 and the other years returning on January 18.
Who will go back to school when?
The 'majority of primary schools will return on January 4 except those where infection rates are highest.
Years 11 and 13 of secondary school will return on January 11.
All other years and college and university students will return on January 18.
Where schools are closed, only vulnerable children and children of critical workers will attend face to face lessons.
Mass testings will begin in the first week of schools for secondary and college students and teachers.
There has been growing concern from teaching unions and scientists about the spread of the virus following the discovery of its much more transmissible variant.
Mr Williamson said keeping schools open is "uppermost in our plans" and it was a “last resort” that some schools needed to close where infection rates are highest.
He added: “We’ll be opening the majority of primary schools as planned on Monday, January 4. We know how vitally important it is for younger children to be in school for their education, wellbeing and wider development.
Watch the announcement in full:
“In a small number of areas where the infection rates are highest we will implement our existing contingency framework such as only vulnerable children and children of critical workers will attend face to face.
“We will publish this list of areas today on the GOV.UK website. I’d like to emphasise that this is being used only as a last resort. This is not all Tier 4 areas and that the overwhelming majority of primary schools will open as planned on Monday.”
Areas where primary schools will not open as planned:
Barking and Dagenham
Hammersmith and Fulham
Kensington and Chelsea
Southend on Sea
Tonbridge and Malling
Mr Williamson said that "mass testing" of schoolchildren will begin "in earnest" in January, with pupils in exam years at the head of the queue.
He told MPs: “All pupils in exam years are to return during the week beginning January 11 with all secondary school and college students returning full time on January 18.
"During the first week of term on or after January 4, secondary schools and colleges will prepare to test as many staff and students as possible and will only be open to vulnerable children and children of key workers."
He added: "The 1,500 military personnel committed to supporting schools and colleges will remain on task providing virtual training and advice on establishing the testing process with teams on standby to provide in-person support if required by schools.
"Testing will then begin the following week in earnest with those who are in exam years at the head of the queue.
"This is in preparation for the full return of all pupils in all year groups on January 18 in most areas."
Universities are being asked to reduce the number of students returning to campus at the start of January to prioritise students who require "practical learning to gain their professional qualifications," Mr Williamson said.
“All university students should be offered two rapid tests on return in order to reduce the chance of spread of Covid,” he added.
The Education Secretary said the Government expects to deliver 50,000 devices to schools across the country on January 4 to support remote and online learning, adding 100,000 devices in total will be delivered during the first week of term.
He said of schools: “Our best line of attack is to keep them open using mass testing tools that we now have available so that we can ensure children are able to continue to gain the benefit of a world-class education.”
Mr Williamson also said he was “more determined than ever” to ensure children “do not have to pay the price for beating Covid”.
Teaching unions have urged the government to delay the reopening of schools to safeguard against coronavirus infections and to help schools risk assess to ensure Covid safety measures are in place.
Dr Mary Bousted, joint general secretary of the National Education Union (NEU), said she was “astonished” at Mr Williamson’s announcement.
She said: "With warnings from eminent scientists of an ‘imminent catastrophe’ unless the whole of the UK is locked down, and with more cases in hospitals than ever before and our NHS facing an enormous crisis, the Secretary of State is sending the majority of primary pupils and staff back on Monday to working environments which aren’t Covid secure.
"The Government has not, despite being repeatedly asked, published the scientific guidance on the risks involved in school and college reopening. This information is desperately needed – particularly as the new variants of the virus are 50% more transmissible.
"The Government in Scotland will not reopen schools till 18 January at the earliest. The Government in Westminster should have done that at least.
"A longer period of online working for all primary, secondary and college students could suppress virus levels and buy time both for the roll-out of the vaccine and to put in place measures that can keep schools safer."
She added the NEU does not believe schools are “safe enough” for staff to work in and posed a series of questions about soaring rates among pupils as well as the Government’s plans for mass testing.
She said: “We would like Gavin Williamson to explain, if schools are not centres of transmission, why school-age pupils are now the most infected age groups?
“Why is it that primary-age children are the second highest infected of all age groups, or that levels of infection amongst secondary pupils have multiplied by 75 times since the start of September?
“Serious questions also have to be asked about the Government’s plans for lateral flow testing in secondary schools, in particular about the effectiveness of these tests in identifying Covid infection in young people who are highly likely to be asymptomatic, with the tests being supervised by non-medically trained volunteers.
“We do not think it likely that these tests alone can make our schools Covid-secure nor protect the communities they serve.
“We believe the Government’s steps will fail, that cases will continue to rise and that the question of school opening will have to be re-visited but in a worse situation than now.”