More than 100 schools across England were ordered to close buildings on Thursday because they were constructed using unsafe concrete, the department for education said in a statement Thursday afternoon, a few days before the start of a new school year for most students.

The affected buildings contain reinforced autoclaved aerated concrete, or RAAC, a lightweight concrete material that was used between the 1950s and mid-1990s and has been determined to be prone to failures and crumbling.

In all, 104 schools were ordered to close buildings. The concerns about the concrete have been known for years, and schools had been told to prepare for the possibility that evacuations would be necessary at some point. One hundred fifty-six schools were confirmed to have used the lightweight material in their buildings, but 52 have put in safety measures to mitigate the risks.

The government said it would work with local authorities on “individual solutions” for the affected schools. That could mean using other buildings for classes, sharing space with other schools or, in some cases, erecting temporary buildings. Online classes are a last resort, the government said.

England’s education secretary, Gillian Keegan, said the government was trying to be vigilant for the safety of students and staff.

“Nothing is more important than making sure children and staff are safe in schools and colleges,” Keegan said in the statement, adding that was why the government chose to take action now, at the start of the school year.