Pressure is growing on the government to keep all schools in England closed for two weeks after the Christmas break amid a surge in coronavirus cases.
Teaching unions have told primary school staff it is unsafe to return to work, and called for remote learning.
Head teachers have begun legal action to force ministers to reveal data behind the decision for some schools to reopen on Monday.
The government said decisions are based on new infections and NHS pressure.
Labour has accused the government of "creating chaos" for parents.
Most primary schools in England are expected to open on Monday while secondary schools will reopen on a staggered basis, with exam year pupils returning on 11 January and others returning a week later.
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On Friday, Education Secretary Gavin Williamson announced that all of London's primaries would remain shut on Monday - reversing a decision to keep only schools in certain boroughs closed.
He said the closures were a last resort in the face of a fast-moving situation.
Elsewhere, Brighton and Hove Council has advised primary schools to switch to remote learning next week.
Meanwhile, president of the Royal College of Physicians, Prof Andrew Goddard, told the BBC the new highly infectious strain of coronavirus was spreading across the country, adding: "All hospitals that haven't had the big pressures that they've had in the South East, London and south Wales should expect that it's going to come their way."