The government has no record of how many schools have off-rolled pupils in the last academic year, Tes can reveal.
And the figure "cannot be estimated from current data sources", according to the schools minister.
A teachers' leader called the comments "unacceptable", especially given that a major investigation conducted by the Education Policy Institute (EPI) two years ago produced "clear, well-valued evidence about the scale and extent of this problem".
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The Department for Education (DfE) is now being urged to "provide an update" on "where it has got to" with tracking pupil moves.
The admission from Nick Gibb comes two years after the government consulted on a proposal for a register of children not in mainstream school, maintained by local authorities.
Former education secretary Damian Hinds said at the time that the register would "prevent vulnerable young people from vanishing under the radar".
But the DfE never published the outcomes of the consultation, and the register never came into existence.
Tes understands that the department still plans to share the consultation response, but no date for this has yet been confirmed.
Mary Bousted, general secretary of the NEU teaching union, told Tes that the delay of nearly two years was "ridiculous".
"The only conclusion that one can draw from that is that the government doesn't think it's important to have a register of children not in school," she said.
Dr Bousted claimed that Mr Gibb and others "know the increased risk of harm that comes for children who are...not properly registered, where their whereabouts are unknown by responsible bodies".
She added: "So the question has to be: Why are you not interested? It's like Waiting for Godot. We're used to the Department for Education being slow but that's ridiculous."
Shortly after the DfE consultation launched in April 2019, the long-awaited Timpson Review of school exclusions was published. It called for increased "transparency" of "when children move out of schools, where they move to and why", adding that "pupil moves should be systematically tracked".
The Association of School and College Leaders (ASCL) is now calling on the government to "provide an update over where it has got to in this process".
The review, led by former education minister Edward Timpson, said the DfE's decision to consult on a "new register of children not in school" was "welcome".
"It is essential that LAs know how and when children move around our school system, and DfE have oversight of this. This includes knowing where the child is, who is delivering their education and why a decision has been made to move them," it said.
"All schools, including independent schools, already have to report to the LA when a child is added to, or removed from, the school register, including moves to education otherwise than at school, such as to home education.
"However, this is often not granular enough to understand how or where they have moved, and there is not enough oversight or ownership of this information and some LAs report it simply is not forthcoming at all.
"It is a welcome announcement that DfE is consulting on a new register of children not in school, which will – for the first time – mean that it is possible to identify, on a systematic basis, where children are, if they are not in school."
Two years on, in response to a written question lodged by education select committee chair Robert Halfon, Nick Gibb has admitted that the government does not know how many schools have off-rolled pupils in the last year, and cannot work this out using existing data.
Asked "what recent estimate his department has made of the number of schools that have off-rolled pupils in England in the last academic year", Mr Gibb said: "The information requested is not held by the department and cannot be estimated from current data sources."
He added: "The government is clear that off-rolling is unacceptable in any form. The department will continue to work with Ofsted to define and tackle it."
But Dr Bousted said the admission was "unacceptable", and showed that "the government's priorities are wrong".
"The government should be very, very concerned that the likelihood is you're more likely to be illegally excluded from education when you most need it. Surely that should be a concern," she said.
"This is a state-funded education service, we should be educating all the nation's pupils. And for Nick Gibb to just glibly say, well he doesn't know and he hasn't asked, is unacceptable, particularly as there is clear, well-valued evidence about the scale and extent of this problem."
She added: "This should be a key priority, that the government knows whether children, particularly vulnerable children and young people, are in school where they're safest."
In his response to the written question, Mr Gibb went on to say: "Ofsted already considers records of pupils taken off roll and revisions to the framework in September 2019 strengthened the focus on this.
"Where inspectors find off rolling, this will always be addressed in the inspection report and, where appropriate, could lead to a school’s leadership being judged inadequate.
"A pupil's name can lawfully be deleted from the admission register only on the grounds prescribed in Regulation 8 of the Education (Pupil Registration) (England) Regulations 2006 as amended.
"All schools must notify the local authority when a pupil's name is to be deleted from the admission register under any of the grounds prescribed in Regulation 8. This should be done as soon as the ground for removal is met and no later than the time at which the pupil's name is removed from the register."
Geoff Barton, ASCL general secretary, told Tes that gathering data on off-rolling is "extremely difficult", as it is only possible to establish whether it has taken place "through an investigation".
However he said the Timpson Review came up with a "sensible suggestion" to track pupil movements in "more detail", in order to "understand how and where pupils have moved".
"This would capture legitimate choices made by parents but it would also allow greater scrutiny and action where concerning trends are identified," he said.
Mr Barton added: "The report recommended that local authorities work with schools in reviewing this information, identifying trends, and taking action where necessary. It also suggested that the DfE should have oversight of this.
"The DfE does need to provide an update over where it has got to in this process, and indeed on other aspects of the Timpson report.
"This isn't because we think that off-rolling is happening widely. On the contrary, our experience is that most school leaders view it as an abhorrent practice.
"However, having this level of clarity over pupil movements would ensure that unusual trends are identified and investigated, and it would provide reassurance to parents, pupils, and the wider education community."
The DfE has been approached for comment.