U.N. Secretary-General Antonio Guterres warned on Tuesday that the world faces a “generational catastrophe” because of school closures amid the coronavirus pandemic and said that getting students safely back to the classroom must be “a top priority.”

Guterres said that as of mid-July schools were closed in some 160 countries, affecting more than 1 billion students, while at least 40 million children have missed out on pre-school.

This came on top of more than 250 million children already being out of school before the pandemic and only a quarter of secondary school students in developing countries leaving with basic skills, he said in a video statement.