RAAC: BBC expert answers your questions
The government has published an official list of 147 schools in England it says have crumbling concrete called RAAC.
The Department for Education says nearly 3,000 pupils are going back to learning from home and another 22,000 will do a mix of home and face-to-face learning.
There's no lists for the rest of the UK, but two school have been closed in Wales, 35 schools have been identified to have buildings containing the problem concrete in Scotland, and schools in Northern Ireland are still being checked.
BBC Education Reporter Vanessa Clarke came into Newsround studio to explain why RAAC has been causing safety problems for schools.