Teacher Francie Keller welcomes the pupils of class 3c in the Lankow primary school to the first school day in Schwerin, Germany, Monday, Aug. 3, 2020.

Video. COVID-19: Over 1 billion students affected by school closures

United Nations Secretary-General Antonio Guterres said the world is facing a "generational catastrophe that could waste untold human potential, undermine decades of progress, and exacerbate entrenched inequalities."