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 this as a "scramble" coming only days before the start of the new term.
Some children face learning remotely or using temporary classrooms.
The government said the decision followed "new evidence" on the material.
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 "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.
