"A worryingly low number" of vulnerable children allocated a school place to keep them safe during the coronavirus crisis are actually turning up, officials have told BBC Newsnight.
In some areas just a quarter of the "at-risk" children who are meant to be in school are believed to be attending.
The figure is thought to be below 10% in some parts of the UK.
These low attendance rates are concerning professionals working to protect these children.
Norfolk's Chief Constable Simon Bailey, the National Police Chiefs' Council lead on child protection, told Newsnight: "Is it possible that we will see a coronavirus impact upon child sexual abuse?
"Yes I think it's possible, in exactly the same way as I've got to work on the premise that we will also see more children groomed and abused online."
The school attendance figures for Norfolk's vulnerable children are 13%, Chief Constable Bailey said.
"It is a worryingly low statistic at the moment. When a child is in school, in the vast majority of cases they are safe, they are protected," he said.
"Of course there will be any number of reasons why those children are not there. Some might have underlying health issues and their parents are being completely responsible by not putting them into school.
"But I think there are a number of risks and I fear that children are being exposed to the potential for familial abuse within the household, being exposed to domestic abuse again in the household and we have seen a significant increase in reports of domestic abuse nationally."
Newsnight spoke to a school social worker, on condition of anonymity, who said: "In one school I work with, not a single child attended who was supposed to be there last week.
"A good turnout in the schools I work with is under 10%. It's quite frightening.
"The people who deliberately hurt children… for those people, this works very much in their advantage because they do have quite literally a captive audience, they have people there who can't get away."
Schools are submitting their figures to the government on the numbers of vulnerable children still attending school.
Newsnight asked the education departments for England, Scotland, Northern Ireland and Wales how many education places they had allocated for vulnerable children and how many were actually turning up.
England's Department for Education said it was not publishing the figures, while the others could not supply them.
England's Department for Education told Newsnight: "Being in school can keep vulnerable children safe and ease pressure on families, which is why we have enabled these children to continue attending despite schools being closed for other pupils.
"We thank schools and social workers for the work they are doing to keep vulnerable children safe at this challenging time.
"We have provided guidance to help ensure that removing risks to children remain at the forefront of all efforts during this pandemic and we are supporting social workers - including those returning to the profession, to help on the frontline - to focus on those in the most urgent need."
Watch the episode of Newsnight on BBC iPlayer here.