Two in five schools do not provide children with soap and water, according to a report by the World Health Organization (WHO) and UNICEF.
In the poorest countries, seven out of 10 schools lack handwashing facilities, and half do not have access to basic sanitation and water services, the study found.
More than a third of the roughly 818 million children who lack these basic facilities are from sub-Saharan Africa.
The report comes as schools around the world are struggling to reopen during the ongoing pandemic.
"Access to water, sanitation and hygiene services is essential for effective infection prevention and control in all settings, including schools," WHO chief Tedros Adhanom Ghebreyesus said in a press release.
"It must be a major focus of government strategies for the safe reopening and operation of schools during the ongoing COVID-19 global pandemic," he added.
The coronavirus outbreak has created the largest disruption to education ever, affecting nearly 1.6 billion students in more than 190 countries, according to the U.N.
WHO and UNICEF stressed that governments must weigh public health measures to curb the virus against the social and economic impacts of lockdown measures, as children's education and wellbeing suffer under prolonged school closures.