Thousands of pupils in England face disruption after more than 100 schools, colleges and nurseries were told to shut buildings with concrete prone to collapse until safety work was done.
School leaders described the news as a scramble, with it coming just days before the start of the new term.
Some children face learning remotely or using temporary classrooms. The government said that the decision came after new evidence on the material was revealed.
Schools with reinforced autoclaved aerated concrete (RAAC) must introduce new safety measures which could include propping up ceilings.
Until this is done children may have to be moved into temporary classrooms. Two primary schools in Bradford - Crossflatts and Eldwick - are among those affected, with parts closed to pupils after the concrete was identified, the council said.
The Department for Education (DfE) said that any space or area with confirmed RAAC should no longer be open without mitigations in place. It has not given a timeline for replacing the RAAC, or named the places affected.
Watchdog, the NAO (National Audit Office), recently highlighted concerns for school buildings that still contained RAAC. This is a lightweight 'bubbly' form of concrete used widely between the 1950s and mid-1990s - usually in the form of panels on flat roofs, as well as occasionally in pitched roofs, floors and walls. It has a lifespan of around 30 years.
While the vast majority of schools and colleges will be unaffected by this announcement, the NAO report identified 572 schools where this concrete might be present. There are 156 settings in England with confirmed RAAC, according to DfE data. Of those, 52 already had safety mitigations in place, and 104 were being contacted this week about getting them in place.
Education Secretary Gillian Keegan said: "We must take a cautious approach because that is the right thing to do for both pupils and staff.
"The plan we have set out will minimise the impact on pupil learning and provide schools with the right funding and support they need to put mitigations in place to deal with RAAC". Julie McCulloch, director of policy at the Association of School and College Leaders, which represents mostly head teachers, said: “The government failed to invest sufficiently in the school estate". She described the announcement as a "scramble", saying it was "clearly vital", but "the actions these schools will need to take will be hugely disruptive, and this will obviously be worrying for pupils, families and staff".
She added: "The government should have put in place a programme to identify and remediate this risk at a much earlier stage." Bridget Phillipson MP, Labour's shadow education secretary, said ministers had "been content to let this chaos continue for far too long".
Liberal Democrat education spokesperson, Munira Wilson, said: "Pupil safety is paramount but for this to come out just days before term starts is totally unacceptable". The government says it has been aware of RAAC in public sector buildings, including schools, since 1994.
It said it has advised schools to have "adequate contingencies" in place since 2018, in case affected buildings needed to be evacuated.
