Experts say there is not enough evidence to reopen schools yet because it is still unclear how coronavirus spreads among children.
Schools and colleges have largely been closed under lockdown, except to children of essential workers and vulnerable youngsters, but it is thought they could be among the first places to reopen once restrictions start being eased.
But chief medical officer Professor Chris Whitty said that while a “great majority of children” either do not contract the virus or have mild symptoms, there is still not enough data on how youngsters contribute to the spread of Covid-19.
He said the lower the transmission rate of coronavirus – or R value – the “greater room for manoeuvre” there is for what lockdown measures could be lifted.
Speaking at the Downing Street briefing on Monday, Prof Whitty said: “If you have schools open, it does contribute to increasing the R.
“If you close schools, the R goes down, it was part of the collection of things that were done in March to try and pull the R from where it was, near three, to where it is now, below one.”
He added: “If you stopped doing it, you would actually lose some of the benefit that we have currently got.”
But “we do not yet have direct data that really help us” in assessing the impact, he said.
There is more accurate information on how children spread diseases such as flu, Prof Whitty said, but “we are still really learning” on how they spread Covid-19.
There will have to be some difficult choices, and choices around schools clearly will be one of thoseProfessor Chris Whitty
He added: “Whilst I think it remains the case that the contribution of children at school is probably less than, for example, flu, we do think it certainly contributes.
“What we are trying to work out is what proportion of the R it contributes and therefore, if children went back to school, how much closer to one – and that’s in a bad way – would we be and could it even tip us above one.”
He said scientific advisers would present a range of options to ministers about easing lockdown measures and “difficult choices” would have to be made.
Prof Whitty added: “There is no perfect solution where we are going to end up being able to do all the things that people want and, at the same time, keep R below one.
“So, there are going to have to be some very difficult choices between different things, all of which ideally all of us would love to open up, but we can’t do them all.
“Therefore, there will have to be some difficult choices, and choices around schools clearly will be one of those.”
When asked whether there is a plan on opening schools, Health Secretary Matt Hancock said it was “too early” to make a decision and urged the public to follow social distancing rules.
It comes after Education Secretary Gavin Williamson previously said the Government had “no plans” to open schools over the summer period, while Foreign Secretary Dominic Raab said it would be “inconceivable” without some further measures in place.
Head teachers have previously been advised to start making preparations on how schools could safely reopen, with the Association of School and College Leaders (ASCL) suggesting a staggered return of pupils, with Year sixes, Year 10s and Year 12s phased in first, if permitted.